XP System Hanging during Software Downloads

An XP system I have been working on continually hangs when trying to add or remove software. I have tried real simple installs over the broadband connection but it never finishes. Even a disk cleanup doesn't finish (trying to remove 2gb of data). When I try to remove certain hardware from Control Panel -> Add/Remove Programs, it hangs. The only way out of these is to kill the task or perform a hard reboot. I'm afraid the system is hindered by adware, spyware and viruses but I cannot download anything to help kill of these programs. Can someone give me advise on what to do next?

J-