Hey there and Welcome to CDFs!
Valve patched HL2 along time to remove the DVD check (SecuROM 5), so backing it up isn't a problem. I would say the best program to use is DVD Decrypter. Google it and I'm sure you will find a download mirror. It is illegal in many countries now (well one ability of it is) but that shouldn't stop you using it. Once you have it installed, fire it up and select: Mode > ISO > Read. Read the disk off into a safe place (like your desktop). Then select Mode > ISO > Write. Tell it where the ISO is, put a blank disk in the target drive, then hit the big button! Good luck.
P.S. If I remember right, SecuROM 5 disks have a single read error @ the end of the disk. If it does just tell DVD Decrypter to skip that sector.