One thing which is worth a shot, if you are using XP SP2, is to disable DEP. I found this caused me to have weird wall-hitting effects. (I could clock really well in 2k, and in XP SP1, but not SP2 - like 2.9 Ghz in 2k, XP SP1, but only 2.6Ghz in XP SP2).
To disable DEP, change the boot.ini from this:
/noexceute=optin
to this
/execute
It's unlikely to work, but it did for me once, and you can always change it back.