NO!
Do NOT re-install windows. That is by far the
STUPIDEST suggestion ever. If you're running NT/2000/XP, then just do the following. It will make your day

brighter, and save yourself hours of program re-installations.
Start -> Control Panel -> Administrator Tools -> Computer Management
Click on "Disk Management" and locate your new hard drive in the list. Check to see if there is a partition listed on the drive. If not, simply create one, format it, and then restart windows.
If you're running Win9x, simply use the software provided by Western Digital (also downloadable from their website) and all will be well.
I swear people, if you don't know what you're talking about, don't bother even posting a suggestion.