To do a repair install:
When you boot to the CD it will load some files, then give you the options:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
or
To quit Setup without installing Windows XP, press F3.
DO NOT press R. Instead press ENTER to continue with Windows XP setup. It will give you the EULA (end user liscense agreement), accept that. Your computer will then scan for existing installations of windows. After it locates your current windows installation it will give you another chance to repair it.
Highlight your current installation and press R to repair it. If repairing it is not an option, then quit right there or you risk loosing all info on your PC (it won't do it on it's own, but you will be one step away so don't press the wrong button).
This will reinstall windows over your current installation (leaving any viruses you may have had still on your system). It should give you access to explorer.exe again though. If you do this, and it works, then I would look over the
First Steps at Removing Malware