My in-laws' computer is crawling with Trojans. Help!?

After their computer slowed to a crawl, they asked me to look at it. I installed a security suite which works great on my computer. It found 4 viruses, which could not be deleted, disinfected, or quarantined (apparently they're running in the background). I attempted to run the virus scan in Safe Mode, but the program wouldn't run. I know the names of at least two of the worms: Trojan-Downloader. Win32. Small. dkt and Trojan-Proxy. Win32. Lager.az. What are my options?