You've installed a firewall and anti-virus tool, but does that mean that you're safe online? Unfortunately, no. Spyware, browser hijackers and another annoyances still have many ways to infest your PC, and that's why it's worth having a dedicated spyware tool to remove them.

There's no particular need to pay for one, of course: comprehensive tools such as Ad-Aware SE Personal are available for free. So we wondered if Spyware Doctor was so effective that we'd be persuaded to part with our £16. We ran the two against each other to find out.

The bare statistics of our first test didn't look good. Spyware Doctor has fewer signatures in its database (26,840 when we tested, against 30,122 for Ad-Aware). Furthermore, running a Quick Scan in Spyware Doctor took over 25 minutes, while Ad-Aware's Smart System Scan was complete in under six.

Signature totals don't tell you everything. Although Spyware Doctor was slow, that's partly because it scanned more objects (148,507 against 89,816 for Ad-Aware). As it claimed to detect 1,141 problems, while Ad-Aware only managed 146, was the extra time well spent?

To remove spyware successfully, you first have to detect it, and that makes the total number of threats you discover particularly important. Does that mean Spyware Doctor wins, as it lists 1,141 infections detected? Not necessarily, because on checking the scan report, we found that count included multiple entries for the same threat.

So, for example, Spyware Doctor told us that NS Keylogger had been found on our system, and that installation included 215 different Registry keys. It then counted these as 215 of the infections, even though - as far as you're concerned - it's just discovered one program.

Safe or suspect?

Then we decided to check whether or not this keylogger was installed at all. On closer examination that's not entirely clear. Spyware Doctor identified it as present in what we believed to be a legitimate file called jmail.dll, and its associated Registry entries. Ad-aware didn't pick this up, Norton AntiVirus saw no problems here and Googling for the ActiveX control class IDs didn't reveal anyone else thinking they were a risk.

Furthermore, another innocent program was identified as backdoor Trojan IRC.Comiz by Spyware Doctor but not Ad-Aware or Norton AntiVirus. It began to look like the scans weren't entirely accurate. Could Spyware Doctor win us over with its final trick?

Most spyware detectors rely on you remembering to run a scan occasionally, and otherwise leave you entirely unprotected - not this one. Spyware Doctor also includes OnGuard, which optionally monitors your PC at all times, blocking infections almost as soon as they happen.

This sounds as if it's a good idea and there's a reasonable mix of tools involved: checking for new malignant cookies, programs installing themselves to run at Startup, or known spyware processes from launching at all. There's also a pop-up blocker and keylogger detector.

Unfortunately, the concept is spoiled by poor implementation, with Spyware Doctor simply running a host of file and Registry checks over, and over again. The end result is that every few seconds Spyware Doctor consumed 50 per cent of our CPU resources, reducing everything else to a crawl. You could limit this by reducing the checks ActiveGuard makes, but it's always going to be a resource hog at heart.

The final blow was when we noticed that when Spyware Doctor was running, SmartStore.biz (an e-commerce tool) no longer displayed its main screen properly. It uses an embedded browser window, which Spyware Doctor appears to block in some way. Hassles like this can be a nightmare to diagnose, and if reproduced with similar programs suggest Spyware Doctor could be far more trouble than it's worth. Mike Williams