Even though a format and reinstall can often be more time efficient, cleaning the system should give the owners an idea what to stay away from in the future.
In addition to the A/V get Superantispyware for periodic malware scans. If they already have many viruses they probably have a lot of everything else that the viruses and trojans could download.
They will need winsockxp.exe (get it before running scans) also in case their internet access is broken by removing the malware.
Update the A/V and Superantispyware and run them in safe mode. That should give them a good start and may fix them totally. If they still have problems or can't run the programs there are more steps that can be taken, but start there.
Jim