Well...you can try a repair install of XP...but you may ultimately find yourself at the Clean Install Window.
IMO, removal of malware items prevents the system from incurring additional
damage/changes...but it does nothing to reverse effects that have already taken place
or which take place as part of the removal process.
I'm not a malware expert, just a user...but my premise makes sense to me.
How to perform a repair installation of Windows XP if Internet Explorer 7 is installed - http://support.microsoft.com/?kbid=917964
How to Perform a Windows XP Repair Install (Stevens) - http://www.michaelstevenstech.com/XPrepairinstall.htm
An alternative to attempting a repair install is to try to run the sfc /scannow command, How To Use Sfc.exe To Repair System Files - http://www.bleepingcomputer.com/forums/t/43051/how-to-use-sfcexe-to-repair-system-files/