Strange mouse bug only in source games. Formating drive does not help.

I have struggled with this problem for over 18 hours now. A troubleshooting session that culminated in reformating my hard drive, and reverting my windows 7 RC installation to windows XP. Unfortunately, it did nothing to help.

While Im playing source games, my mouse does not work properly. I have no idea what is causing this, or how to even explain this right, but while playing source games my mouse feels... unresponsive. Certain turns dont execute properly, and when I walk I can sometimes see some kind of weird micro stuttering (like the screen jumping ever so slightly, or a choppy walk through some terrain, despite my FPS recording showing a steady 60 FPS on games like L4D).

Ive not even installed logitech setpoint, for fear of this causing this odd issue; yet still, it happens. CoD4 works fine, Killing Floor works fine, source games now, for whatever reason, have ridiculously unresponsive mouse controls.

At first, the problem was so bad, that the mouse barely responded at all to my commands, at the end, its subtle, but still there as I attempt to play my usual classes and get destroyed whilst doing so. Even replacing the mouse (Logitech G7) with a different one (Razer Copperhead) has NO EFFECT AT ALL. It's 5:22 in the morning, my stomach feels like it's going to implode, the sun is rising and the birds are beginning to sing; for the love of God, has anyone even had something like this happen to them before? What can I even do to fix this? THIS IS A FRESH INSTALLATION OF WINDOWS XP Service Pack 3. ALL DRIVES HAVE BEEN REFORMATTED AND THIS PROBLEM STILL PERSISTS, NO MATTER WHAT MICE I USE.

My peripherals are as follows : Logitech G15 keyboard, Logitech G7 cordless mouse, Nostromo N52 speedpad.
My graphics card is as follows: ATI 4890 OC (XFX)
My operating system, Windows XP SP 3
4 Gigs of ram, 3.00 Dual core processor
My motherboard: MSI P35 Platinum combo
My power suppy: Corsair 650w
My sound card: XFI soundblaster Xtreme Gamer

The mouse works FINE everywhere else, source games seem to be the only thing affected. Namely, L4D and Team Fortress 2. All other games, including Mass Effect, and CoD4 worked FINE. No weird stuttering, no imprecise mouse.