First: Go off-line. If you suspect a virus has struck your computer — it suddenly starts working very slowly, crashes often, or seems to be doing something even when you aren’t — your first step should be to “disconnect your computer from your Internet provider to keep it from spreading,” says Silas Miller, president of Rent- A-Geek, in Burlington, Vermont.

Then: If you have antivirus software, such as Norton’s or MacAfee’s, run a scan immediately. If the software detects a virus, it will take action to get rid of the virus and notify you when it has. If you don’t have antivirus software (or if yours is out-of-date), call your computer’s manufacturer for phone-based assistance, says Gretchen Miller, director of product communications for Dell. And “if you have a second PC or a laptop, use the uninfected one to get advice from the manufacturer’s website on how to fix a virus,” she says. If all else fails, step away from the machine and call a pro.