If you're on Win 7, go to C:\Users\your_username\AppData\Roaming\Bitcoin then backup the wallet.dat file.
EDIT:
Or if you're on Win XP, go to C:\Documents and Settings\your_username\Application data\Bitcoin then backup the wallet.dat file.
After making sure that you have successfully backed-up your wallet.dat file (at least make 2 backup copies on different mediums ie. flash drives and in a folder in your hard drive) from the directories that I have mentioned above, delete everything inside the Bitcoin folder, then paste a copy of the wallet.dat file inside that Bitcoin folder (from where you copied that file) and then run the bitcoin-qt client again. You'll have to wait for the blockchain to be downloaded again to view the total number of bitcoins you own.
hashkey explained it well, but it is worth repeating...
The wallet.dat file has all of your private keys in it. If you lose the most recent good copy of that file, you will probably lose bitcoins. If you loose all copies of that file you will lose all of your bitcoins. If you think you made a copy of the correct file, but you made a mistake and copied the wrong file you will lose all of your bitcoins.
As long as you have a recent enough good copy of wallet.dat (or a valid list of all the private keys), your bitcoins will be recoverable.
One last warning:
Be very careful when doing anything with your wallet.dat file. If you damage or lose it, you will lose your bitcoins permanently.
