There was issues before with couple of AV programs and when I used Vista, my ET didn't work properly. With Win7 pro I didn't install those at all and all is fine. I have 3 ET installed, both 2.60b and ETL and one is from memory stick.
AVG and Avast, both free versions were the issue. Are you using those maybe?
Another I could think of, is that uninstall leaves folders and maybe registry marks behind. Try using for example CC cleaner after uninstall and then make a fresh install.
Other than these, I raise my hands.