Are you getting the exact same problem in other games?

If not, you know the problem is with ET, not the mouse/computer.

I'm saying this because I read you tried with new mouse and a whole new computer as well.

Also: have you tried with another mouse pad?

Install windows 7 :D if your pc does not support windows 7 then;

Reinstall Windows Xp with different version (home/pro/media center etc). Do not install anything else except drivers. (No antivirus no setpoint etc)

Dont use any accel patch just start ET and play normal.
If you feel no difference then change your config. Do NOT use your regular config. Try some one else config e.g. mAus config ;)

If you feel no difference then its your pc.
Thank me afterwords ;)

"Try ... mAus config ;)"

- oh my god

Guys, I'm not ratzilla but i have the same problem. It's not a problem with the game, because the feelin of my mouse is weird also in win xp. For example suddenly my mouse starts to work great, and I'm afraid to make a reboot :D because it will work so weird after it =x I will try with win7. I made some tests with my hardware and I didn't notice any errors =/