I know we have a fixed price for installs.
We always load Windows, install the drivers, put in all the service packs and any updates from Microsoft (this takes a LOT longer than just running the install disc) and then put a freebie AntiVirus & Anti Malware prog on.
If its dirty inside it also gets a clean out and a new CMOS battery (amazing how many faults that cures).
Depending on the specs of the machine if your going to pay a shop to do it you can sometimes be better off seeing if they have anything second hand. Last week we had a guy come in wanting a reinstall on an old desktop P3 with 256MB and a 40GB Hard Drive. We could have done it (and made a bigger profit) but to me it wasn't worth it when we had a used machine with a much better processor, 4x the memory and a bigger Hard Drive for more or less the same price, which was all installed ready to go. Even put his old drive in as a slave.
With the older stuff it's sometimes better (cheaper) to play around with it yourself to learn, and if you mess it up completely, it doesn't matter, you just know not to do that again