If you have original WindowsXP CD then you can try this:

0: >!!!> you must have adminstrator rights in order to to that
1: ) you can also login as administrator (logout to welcomescreen and push "ctrl+alt+del" 2 times then write Administrator, by default no password is requid)
2: ) clear dllcache "%windir%\system32\dllcache"
3: ) find and delete msconfig.exe (cmd > del msconfig.exe /s)
4: ) use CD if windows will aks for CD to restore files, if CD is not oroginal or is in wrong version you may got some problems.
5: ) follow instructions in windows and reboot computer
6: ) do full scan of computer with your antivirus and anti-spyware, or download and run "SpyBot" and "ClamWin" if your not working or if you have no one.
7: ) go to windows update site and install updates or use windows update from control panel (check "download and notify when download is complete")