1. Listen to the problem in a dead serious face and make comments like "windows suck" or "seems like a Trojan".
2. Ask them to restart it.Normally solves half of the problems.
3. If it has something to do with Internet explorer - ask them to clear the cache and cookies. Works like wonders.
4. Go through the steps of running a virus checker and a spyware detector.
5. If none of them works - politely suggest contacting the tech support.