system
39
A repair install…not a new install that blows away your HDD… 
http://www.michaelstevenstech.com/XPrepairinstall.htm
Also, prior to the above another thing to try is SFC /scannow …this can repair O/S issue.
http://pcsupport.about.com/od/toolsofthetrade/ht/sfc-scannow.htm