There´s quite a few way you can loose your OS. The most common is indeed overclocking and the most common there is a missing system file.
Well I´ve never seen this guide on XS before so i´ll give it a shot and post it for you guys in case you didnt knew it. I´ve been doing this a number of times and it works everytime.
Error:
System glitches the loading windows screen and show following error:
"System could not start cause of missing file
\windows\system32\config\system. (something)"
Solution:
Insert your windows cd. Reboot on the CD and the first following windows install screen you choose "recovery console"
Once the install enters recovery mode you should enter.

cd\ (the console might start at c:\windows)
cd windows\system32\config (if the console start at c:\windows\ you should only enter the digit´s after c:\windows\)
dir system
ren system system.bak
copy c:\windows\repair\system

Press CTRL + ALT + Delete or hit the reset button
And badabing - Windows is now working. Although you should reinstall chipset and graphic driver I have not notice any performance slowdown after doing this method

m O e