When did this start happening exactly? Did it start happening after updating to a driver? Did it start happening when you installed/uninstalled/etc something else?
I've seen a few things online, one thing that can cause this is conflicting drivers (you used to have an nvidia card in the past/older drivers causing issues still/etc). You can use a driver cleaner to completely remove all video related drivers and then do a clean install.
Another thing I found was this:
"The problem with the PTEs was being caused by a conflict between the built in ATI 3200 HD chip on my motherboard and the new nvidia card... but it shouldn't have, right?
The reason was because in Windows XP x64 Service Pack 2 there was a bug in the videoprt.sys file. If you replace this file with the pre-service pack 2 version, everything works fine.
Good version: 5.2.3790.1830
Bad version: 5.2.3790.3959
So, if anyone else has this problem and finds this post, find the good version of that file and replace it. You'll have to replace it either from a parallel install of Windows or from a boot disk like Mini-PE or a floppy because Windows file protection won't let you replace it otherwise."