I've had a few calls from people like these, and depending on whether I've got some time to while away with them I either cut them short or string them along until they give up. My record for stringing them along was just shy of 30 minutes.
One guy told me my computer had a virus, so I asked if he meant it had a cold, and then asked whether I should put some plastic sheeting down in case it sneezed and made a mess on the carpet. After going round in circles on that one for a few minutes he shifted to spyware, so I asked if the neighbours were using a telescope to watch my wife in the shower, and whether James Bond was hiding behind the door. When he said it was in the computer I commented how there wasn't much space in the computer and I didn't know how James Bond could fit inside it.
Surprisingly he still kept trying to get back to his script but after 20 minutes when he asked me to connect to the internet and I innocently read the "web site can't be found" (I disconnected the router so I could get the precise error message) and said that I went to the library to go online he just hung up.
Sometimes when I string these people along my wife has to leave the room because she's practically wetting herself laughing.
On occasion if I can't be bothered to deal with them I just say something like "I'm glad you called, I downloaded 20 gigabytes of hardcore porn and my computer hasn't been the same since" and ask whether the virus it has is an STD it caught from one of the performers.
I think my favourite line was when the guy told me to turn on the computer, and I said "Ah, Scandisk has just started, do you mind waiting" and just let him wait for 15 minutes while I got on with some other stuff. After periodic comments telling him how far it had got I reached 100% and then noticed how that was stage 1 of 5 complete, and it was just starting stage 2, and did he mind waiting a little longer. He said he'd call back, but sadly I was out when he did. I got home to four missed calls from unidentified numbers, so maybe he did actually call back thinking he had a sucker.