The problem with unplugging a computer at the mains is that Windows does not do a controlled shut down and save all files and so on.
So sometimes your hard disk can become corrupt, causing numerous problems.
Always best, if you can, to do a controlled shut down (but not always possible).
As this is XP it may not have such good recovery capabilities as later versions like Vista or Windows 7.
You could try pressing the F8 key during start up (before Windows loads) so you get a list of options.
One of these options may be repair (I think XP has that option) and see if Windows can do a repair.
Also you can try to go back to the "latest good known configuation" which i think is also available on that list of options.
Also you could try doing a check disk on the hard disk (search the Windows help for "check disk" or "CHKDSK" to see how to run a check disk). This will seach for errors on the disk and correct them.
A While sicne I used XP so cant remember exacly what options there are.