Be prepared that once you touch somebody's PC that anything that goes wrong is your fault.
Here's my latest one -
Bloke rings me to sort out his daughters laptop because it is very slow, so I get it sorted and they are happy.
1 week later he rings me because his internet is slow, so I check mine first on Broadband Speed Checker and it reads 9,860 which I'm chuffed about.
I go round his on his normal PC & laptop and both read 128

After messing with his MODEM I tell him to sort it out with his broadband provider which is the Post Office!
He rings me this morning saying that the problem is down to the laptop I fixed because it all went wrong after that.
I explain that it is also wrong with his normal PC so I go round with my laptop and it also reads 128 on that.
He's convinced (because of the Post Office call centre) that I've broke his broadband with the laptop.
I'm going round tomorrow to talk to the call centre myself.
Other classics include -
After you fixed my PC my printer worked for a month but then went faulty which I believe is your fault.
After you fixed my PC my monitor stopped working after a week.
After you fixed my PC I couldn't install Office on it.
I go round and find it's a warez copy and he hasn't got a serial number.
And so on .......