try a program called Acronis True Image... it has a clone-disk feature that works wonders for stuff like this...
install acronis...
install new drive...
clone from old drive to new drive
remove old drive from machine
boot... should go right into windows
you may need to use something like Partition Magic to resize partitions (although If I Recall Correct, acronis can set this up for you before the cloning occurs)
Verify all your data is there
then I'd wipe the 120GB HDD and buy a $20 external enclosure, put the 120GB HDD in there, and use a program like Sync Back (
www.2brightsparks.com) to make backups of all your important stuff... that way, you're putting the old drive to good use, and you're backing up your stuff