I think its Driver Issue coz i looked around and a few People in the Nvidia forums were also having the Same problem.
Do you have the Latest Drivers installed ?If yes then all you can do is wait for the next release of drivers which can allow you to disable "Write Combining" .
^All i can say is Wait for a newer driver Revision.
Also try using the Xp X64 drivers and then see if u can disable "write combining".
Its RISKY midn you (could cause a BSOD) so pls do not blame me if something goes wrong ..hehe
[U have to "Allow Unsigned Driver Installation" tinstall the XP X64 edition drivers]
Otherwise i suggest u just wait for driver revisions to improve.
There are drivers for vista(built in drivers)
and xp drivers im using now for vista UNIVGA 3.80 are the latest drivers for xp but i always use them on vista.
Hmm......so u are using XP Drivers.
Try these drivers,maybe they will allow you to disble "write combining" since they r Vista specific drivers : Drivers