when I was consulting
I'd typically charge $200 to wipe and reload, assuming no wacky hardware issues that require major additional work because "someone" *cough*theclient*cough* lost all the driver installers. I'd charge by the hour if they wanted the system cleaned up without a nuke and pave treatment, which would sometimes end up costing more (depending on how badly they let the system get hosed and how much time I ended up needing to parse the registry by eye).
As much money as I made at that, I much preferred customers who just used a decent router/firewall appliance, plus less-vulnerable software (Firefox and Thunderbird instead of IE and OE, f'rinstance) and saved me the trouble.
