second

Even if Windows would still boot after replacing the files (though it doubt it since even the file sizes from the two versions are different), it definately won't speed it up. That requires a lot more tweaking than just replacing two boot files

As LLXX said, you should open regedit, then (very important) export the regkey, then remove only that regkey. You can export it by selecting the key (with a folder icon), then go to menu File and choose Export Registry File. If you want to enable the My Computer icon back later, you should import that regkey again into your registry by doubleclicking the file.

I wouldn't think so, cause Windows Server 2003 actually is a complete different OS than XP (though it doesn't look so), and replacing its bootfiles is really risky then. Even replacing changed files from (for example) XP SP1 to XP SP2 is dangerous, and since the kernel version of XP (5.1) is lower than the one from 2003 (5.2), I wouldn't even try this cause your Windows may not boot anymore.