rpcrt.dll error in explorer

I have been asked to look at a freinds system running XP Pro on an Athlon motherboard.It seems on boot up an error message appear re: exporer.exe rpcrt.dll and shutsdown once the error is acknowledged so I tried a bootable CD antivirus scan nothing,safe mode same error,managed to get into recovery console once and put back a couple of days but same error,hooked the drive up as slave on another system and ran AV scans both on the system and online nothing,ran Nortons RPC virus scanner nothing.......so any suggestions welcome.

On further research it may be rpcrt4.dll that is on the error message and the file version installed is 5.1.2600.1254 [xpsp2.030801-1834].