I don't think we have sweeping regulations about it, at least in California.

Most companies I've worked at have a policy of some "reasonable personal use" being permitted. The concern is usually focused on the other way around: Companies do not want their IP on your personal machines.

They can certainly look at whatever is on their own machines, however, regardless if it is your personal data or not.

One large caveat: If you do any work on your company's equipment, they may possibly own it, no matter how relevant it is to the company. It's one of the legal tests used to judge the ownership of your work.