Is there a software I can install on a computer, that locks the computer after a certain time frame?

There's no way you legally charged $75 for parts and labor. A decent hard drive for $75 is pretty hard to come by and windows 8.1 for that is a steal. Together with RAM thrown in? That's pretty impossible if you're using new parts and a legal Windows install.

Therein lies the issue. What you're describing either means you took a loss or you're not legal. If you legally repaired it and used legal parts then you could get away with holding it until they paid you in full. Try to install a virus on their computer to prevent access plus repair it with illegal software and/or potentially stolen parts and all you are getting is sued at best.

So no. I seriously don't see you getting away with trying to force them to pay. If you are completely legal you just gave them about $500 worth of repairs/software for $75 so I think you might be in the wrong business. Seriously you'd barely get me to clean spyware for that price.

How did you replace the hard drive and RAM, install a clean Win8.1 and also a copy of Office 2013 for under $75? That's simply not possible. Even a basic clean copy of Office 2013 costs $120, a clean Windows 8.1 installation costs another $120. Buying the RAM and HD for a decent PC would probably add another couple of hundred dollars onto the price.

Don't return the computer until you are paid in full.

Installing the kind of software you describe, even to enforce a contract for payment, would be a violation of federal law. No kidding.

Your $75 charge sounds like a bargain. Insist on it before you return the computer.

You could write a VBScript that loads at logon, and checks the date.  If the date is before a certain value, the script closes normally.  If the date is after that value, automatically log off.

You'll need to include a trap that allows you to avoid the automatic logoff, for example, look for a keypress such as the left Shift key.