Below I'm posting a step-by-step fix to add TLS1.2 to IE8, so that Skype 7.36.0.150 will continue to run on Windows XP-SP3.  (While 7.38.x.x may be actual "last" for WinXP, it may or not nag you to "update".)    I'm compiling pieces of the fix puzzle I found elsewhere on MSFN, because the complete fix isn't obvious to WinXp Skypers searching from elsewhere on the web.  The fix isn't that difficult, but the usual warnings that novices should back up the registry before editing it, do apply.  The download KB file installs, each set their own restore points.  I hope just setting a Restore point before starting the edit will be adequate.  
I haven't used my desktop PC WinXP-SP3 Skype (mostly chat) for months while the power supply was down.  Yesterday I fixed it.  To my surprise, Skype errored with "Sorry, we couldn't connect to Skype.  Please check your Internet connection and try again."  But the internet was ok. Many Skypers aren't techies, and most of the posted complaints about  "Sorry, we couldn't connect to Skype", don't have a fix other than get a new OS like Win7, or use web Skype.  For good reasons, we don't want to give up WinXP, at least as a backup to Win7 (or even Win8.1 for my keytablet).  I've thoroughly tested Win10, but I'm not interested in that control-freak bugfest. One elsewhere-posted answer with no fix, helpfully explained that Skype had switched to using the more secure https encryption protocol TLS1.2.  Skype for WinXP uses the SSL/TLS protocols built into Internet Explorer 8, which is the last Internet Explorer version for WinXP.  IE8 normally has a maximum version of TLS1.0.  Skype servers apparently turned off insecure TLS 1.0 sometime after I had to quit using this Skype last year (2018).  So the fix is to add TLS1.2 to IE8, and it did work for me.   At MSFN I found the bitter-end holdouts on WinXP, same website where I found the Win98 bitter-enders. (Btw, one poster at MSFN said the famous Windows OS bitter-ender AXCEL216 aka MDGx aka George, is still alive!). One or more MSFN gurus noticed that Microsoft is still updating Windows XP embedded OS for computerized cash registers (etc.), a WinXP variant known as "POSReady" (POS= Point Of Sale).  They figured out how to spoof WinXP-SP3's identity, so that it will pose as, and accept POSReady updates, including those which to add TLS1.2 to IE8.   -----------------------------------------------------
INSTRUCTIONS TO ADD TLS1.2 TO IE8  
   for Windows XP Skype 7.36.0.150
       (Worked for me, but YMMV)
----------------------------------------------------- 1) If not already updated, download and install Microsoft's updated Windows Installer 4.5 (KB942288-v3) from
https://download.microsoft.com/download/2/6/1/261fca42-22c0-4f91-9451-0e0f2e08356d/WindowsXP-KB942288-v3-x86.exe 2) Set a System Restore point marked, say, "Spoof POSReady ID registry edit" 3) Put the following POSReady spoof text (omit the hyphen lines) in POSReady.txt, rename to POSReady.reg, right-click Merge, Yes.
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Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\WPA\PosReady]
"Installed"=dword:00000001
                                                                                [<-- BLANK LINE]
                                                                                [<-- BLANK LINE]
---------- 4) Navigate to: https://www.catalog.update.microsoft.com/search.aspx?q=kb4019276 5) Find down to POSReady, Windows XP Embedded versions of KB4019276 Click Download button for that version. Click English in the opening language window (or other language). 6) Navigate to: https://www.catalog.update.microsoft.com/search.aspx?q=KB4230450
 
7) Find down to POSReady, Windows XP Embedded versions of KB4230450: Click Download button for that version. Click English in the opening language window (or other language). 8) For each KB file: click, accept install, reboot.  (Both create restore points just in case.) 9A) Now edit the following registry entries to read as shown:
(These may be automatic merge .reg texts, but to be careful, I entered them manually.  If you aren't sure how, look up Regedit 5 editing instructions.) 9B) After navigating the chain of registry keys, click the key TLS1.1, in the right panel, right-click "OSVersion", click Modify, enter the Value data shown above, click OK.  (I had to change "3.6.1.0.0" to "3.5.1.0.0" shown in obvious German in the source.)
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[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\AdvancedOptions\CRYPTO\TLS1.1]
"OSVersion"="3.5.1.0.0"
---------- 9C) Next click the key TLS1.2, in the right panel, right-click "OSVersion", click Modify, enter the Value data shown above, click OK.  (Likewise I had to change "3.6.1.0.0" to "3.5.1.0.0")
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[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\AdvancedOptions\CRYPTO\TLS1.2]
"OSVersion"="3.5.1.0.0"
---------- 10) Open IE8, click Tools, Internet Options, Advanced tab, pull the thumb bar all the way down.  You should see new checkbox options for "Use TLS 1.1", "Use TLS 1.2". (KB4230450 will install these checkboxes, but they won't work without KB4019276.) 11) Uncheck "Use TLS 1.0" (insecure). Leave unchecked "Use TLS 1.1" (already obsolete).  Check "Use TLS 1.2".  Click OK.     Now run Skype 7.36.0.150 (similar versions should also work).  When I did this, the "we couldn't connect to Skype" error went away. However, a new sub-login dialog appeared that only allows a Microsoft school or business account.  This dialog went away after I clicked on an existing chat account.  So it may be only an occasional nuisance glitch, perhaps related to help-bot accounts? Pardon any source text compiling errors.  If you have problems, try reading the sources (long).   Sources:  
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https://msfn.org/board/topic/171814-posready-2009-updates-ported-to-windows-xp-sp3-enu/
POSReady 2009 updates ported to Windows XP SP3 ENU
By glnz, March 19, 2013 in Windows XP
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https://msfn.org/board/topic/177500-upgrading-ie8-to-tls-12/
Upgrading IE8 to TLS 1.2
By Thomas S., June 9, 2018 in Windows XP
---------- I hope this helps. Al