chdman.exe built with MAME can do it.
First, plug in your hard drive and identify which device Windows thinks it is (drive 0 will be your boot disk). Once you know the drive index, run
chdman -createhd \\.\physicaldriveX c:\gauntlet.chd
(where the 'X' above is replaced with the drive index, e.g., \\.\physicaldrive2 means the 3rd disk in the system). This will create a compressed copy of the hard drive on your c: drive. Then buy a new disk, plug it in, and extract the CHD file to it:
chdman -extract c:\gauntlet.chd \\.\physicaldriveX
Be careful with this last command as it will overwrite whatever hard disk you specify with the contents of gauntlet.chd.