You’re once-fast computer taking a long time to boot up, freezing or crashing, performing sluggishly or suddenly serving up pop-up ads or finding websites you never pointed to is a sign of spyware, malignant programs that insert themselves on your machine.

If you download a lot of music or software programs such as screensavers, the likelihood is that your computer has anywhere from 10 to 1,000 (not an exaggeration) spyware programs hidden inside.

Security firms will tell you that more than half of today’s computers are infected with spyware.

Fortunately you don’t have to be a techno-geek to give your computer an A-OK health check.

A number of online services offer free spyware-screening programs — but be wary as some can even be spyware programs posing as spyware-killing programs.

Dave Methvin, chief technology officer at PC Pitstop, tells BusinessWeek that the following three free spyware-killing programs are the most reliable: Ad-Aware from Lavasoft, Spybot Search & Destroy, and Microsoft’s Windows Defender.

“If you’ve had [spyware] once, there’s a good chance you’ll get it again,” says Methvin, who advises that once you clean your computer, change your habits to stop downloading suspect software or unknown screensavers. PC Pitstop offers additional info on how spyware and viruses get on your system.

Spyware has become so costly and time-consuming for some companies such as Comcast are offering free services, such as the McAfee suite of virus protection for Comcast high-speed internet subscribers.

McAfee automatically scans documents for viruses and attempts to clean files, protects your computer from unwanted intruders and automatically, checks for updates, and filters inappropriate Web content with Parental Controls, all for free to Comcast subscribers. McAfee also guards against unwanted “cookies,” strings of commands that websites put on your computer for various reasons that can sometimes be malicious.

This is not an anomaly but a trend — look for other large internet and media giants to follow suit with free software protection.