Hope this helps
I'm assuming that you are trying to watch production movies (i.e. Rush Hour) and not home movies. Your DVD-region number may have been reset when you reinstalled Windows. It needs to be Region 1 (if you live in the US. If not, look at your DVD, it should say what region it's encoded for.) To change this (Assuming you are running Windows XP. These steps might work for Windows ME, not sure though):
Double Click on My Computer.
Right Click on your DVD drive.
Select Properties.
Select the Hardware tab.
Double click on your DVD drive.
Select the DVD Region tab.
Scroll down until you find United States (or your country)
Double click it and hit Ok.
You may have to restart the computer for the changes to take effect.
If this doesn't work, try updating your firmware for your DVD drive. Good luck! :grinthumb