[accidentally deleted HKEY_USER .DEFAULT, S-1-5-18, S-1-5-19, S-1-5-19]

So... suggestion probably won't fix missing items.

I had similar problems few months ago, this solution fixed my problem, so you must try it first.

[iexplore.exe not responding]

1)

Start => Run

iexplore.exe -extoff

That command run Internet Explorer in safe mode, without any extensions. Check situation, if IE can connect to internet, you should reset your browser to default.

2)

Reset Internet Explorer to default

Please, read this => CLICK

3)

Flush DNS cache

Start => Run

ipconfig /flushdns

4)

Run SFC.exe to fix broken system files

Start => Run => cmd

sfc /scannow

All broken files will be recovered.

Also, can you advise why this problem has suddnely arisen.

It hard to tell, many possibilities... recently updated system with bad hotfix, malware etc.

[accidentally deleted HKEY_USER .DEFAULT, S-1-5-18, S-1-5-19, S-1-5-19]

Besides, every time I boot my PC, the startup are slow. I have to wait 5min to see and use network connection and be able to see and use my HDDs.

You can try create a new profile and transfer all data from old profile to a new one.

[Fixing booting problems when moving 1 harddrive to another system]

Why just changing the XP Pro harddrive to slave make it unbootable?

You should make that partition active, edit boot.ini file and set properly booting device, and then try boot Windows.

[iexplore.exe not responding]

1) Run IE with safe mode switch (IE7+ only), Start => Run and type iexplore.exe -extoff

2) Reset Internet Explorer to default state

3) Flush DNS cache

4) Run SFC.exe to fix broken system files