Do you guys that do it for a living have some sort of waiver or contract that customers fill out stating that you cannot be held responsible for a virus that attack the hardware or software?Or that by transporting a computer it can be damaged?
Aside from my business accounts, no, I do not have contracts for Home Clients. My business one is half 'you are hiring me to be the only one that even thinks about opening one of your cases or diagnosing a problem for <enter time period>', and half 'I'm not liable for any (potential) missed business through the course of my maintenance on your systems' (meaning, if something they did breaks their network or requires a unit to be in my possession for a day or two, and they lose out on money because of it, I'll not be responsible for the lost sales).
Transit-damage really isn't an issue with me since the majority of Clients I have are 99% on-site, with the occasional take-home job (custom build, after-hours work, etc.).
I was speaking about computers that may already be infected and by trying to remove the virus,,it damages the drive.
Viruses are fun. Many folks will not want to dig too deep to manually remove them. Myself...well, I'm 50/50 on that, it depends on my mood/workload that day. If they have one or two, it's beneficial to them to spend the money to have me track them down and remove them. If it's a heavy infection, it's much easier for me/cheaper for them to have me backup their files, reinstall the OS, and transfer the data. Virus removal can get expensive fairly quick.
AS far as transporting a computer question,,I was speaking about what happens if a customer has a running computer when they leave the house but they unknowingly damage something then try to blame it on you?
Well, that can happen with anything, really; cars, appliances, etc. It happens frequently, but most of the time, the person complaining isn't trying to get one over on you. A common call I get from pick-ups is that they plug it, connect everything, and go to turn it on, but nothing happens. I'll get a call saying 'what did you do?' or 'My system is totally dead!', not knowing about the PSU power toggle

Once I tell them and they fire it up with me on the phone, it's usually an embarrassing moment and they apologize, but it's all good
If you ever got someone who was trying to blame you for something that just sounds 'odd' (you'll get to know the stories they'll tell), I would explain that I take the utmost care in handing and installation of parts, and I would give it a quick visual inspection and diagnostic free of charge. If they're trying to scam you, they'll come up with an excuse why you can't see the machine. Also, the only time I would ever consider reimbursing someone would be AFTER I looked at the machine again, and if I found nothing wrong, I would tell them to take it another PC Repair place, they would be responsible for the bill they gave them, but IF they found either a problem I caused or a problem that I didn't fix, I would reimburse them for the other shop's bill and fix the problem whoever I deemed fit, free of charge.
You may want to buy some security tape - the kind that once it is applied, it gets distorted when you pull it off. That will make it fairly brainless to know if someone's been inside the system after if left your presence. Hope some of this helped!