I think i might have inadvertantly altered the registry on my pc.
When shutting down WindowsXP my system would also shut down.
However when i shut down now, windows closes, but then have to switch off my pc manually.
Any suggestions!!

Check this Microsoft knowledge base article.

http://support.microsoft.com/kb/313290/en-us

This is the simplest case. Most times this problem is related to either power management or USB devices.