First try the System File Checker
Put your windows CD into the CD ROM and hold the shift key (the one with arrow pointing up, just above the Ctrl key on the left of your keyboard). Holding shift for a few seconds will stop the Windows CD from auto running, but the drive will still spin up to read the disk then spin back down again. Once it has spun down you can release the shift key.
Now click Start, and then click Run. In the Run command line type the letters SFC then click OK. This will launch the System File Checker that will sift through your Windows System Files looking for anything that has been replaced by an older version of the one it got from the Windows install.
If it finds any files that have been altered it will tell you and will prompt you on whether you would like to replace the files. If you would, the Windows CD is already in the CD ROM ready for your PC to read the files from, so just follow the prompts.
There is a nice, clear overview of the SFC tool and how to use it at the following link and I suggest you click on it and read about it to make sure you are prepared for what you will see and how to deal with it.
If this does not fix things, post another message and one of us will walk you through other possible fixes.