Many years ago, Logitech issued a keyboard mouse drive that caused such an issue. It was not easy to remove but you can try the usual and hope.
You need to boot SAFE MODE and hope the driver didn't load. Now off to DEVICE MANAGER and "show all devices" then remove all keyboards and mice there.
If you are lucky, it will work on the next boot.
If that fails, it gets deep fast. I have never found a good article about what I do next but http://djlizard.net/2007/04/30/230/ is close enough. I used the same concepts there but on the keyboard entries.
Ok so it all started when I download uninstaller pro to remove a unwanted application. All worked fine until I allowed uninstaller pro to assist with my registry. It was supposed to update or something don't really remember what it said.(which I'm very upset with myself for not paying closer attention like I should) anyways when it prompted me to restart my PC I did and when It came back to the login in screen my keyboard was unresponsive. However my mouse was accessible so I try'ed for a system restore (another big mistake) after the restore finish and returned to the welcome screen both my mouse and keyboard were unresponsive. Oddly however my keyboard works just before windows login screen pops up, I have try'ed everything I can think of to fix this issue such as; unplugging both and plugging back in, restart after restart, changing both mouse and keyboard with new ones, and even tapping f8 to pull up restore still no luck I have no restore Cd so i can't even try that option Please if anyone can offer any advice to this issue It is very much appreciated thank you.
My PC: 2005 Compaq Presario:OS Windows Xp Media center edition
Keyboard: Ps2 and USB model (ps2 is my everyday keyboard. USB is new model I purchased)
Mouse: USB and ps2 (other way around with mouse USB is my everyday mouse and ps2 mouse was purchased as replacement)