So your computer hangs on scandisk? There are three things you can try:
1. boot your pc in safe mode.
You do this by pressing f8 when booting the pc and choose 'safe-mode'. After that, run scandisk in the way silverdown proposed.
2. boot your pc in msdos-mode and then run checkdisk, the dosversion of scandisk.
You do this by pressing f8 when booting the pc and choose 'safe-mode with command prompt'.
Now your screen becomes black and there's something like C:\>_
Type 'cd windows' and press enter
Type 'cd system32' and press enter
Type 'chkdsk' and press enter.
Now your disk is being scanned. If it's finished, press the reset-button on your pc and reboot.
3. if this doesn't work and if you have a legal winxp cd (not a recovery-disk!!) boot with that cd in your cd-drive and choose 'repair existing windows installation'. Now the installer checks if there are important windowsfiles missing and it will put them back.
Don't use a recoverydisk for this because then your system will recover to the state it was when you bought it, so all your documents will be wiped out!
Just one more tip: look for some neighbour or friend with a little computerexperience to help you, and watch what he or she does.
Seems you messed up with the file system. If your disk is not physicaly damaged(I mean bad sector), you should run chkdsk or scandisk from a boot disk or cd. If that is not possible, try to take your disk to a friend, install it as secondary HDD, boot and run the scanning tools from primary HDD.Running the scanning tool from the defective disk itself will not help much.If that also fails, You may end up with re-formatting your disk.