1. Cost
Nearly all anti-spyware packages have a one-off purchase cost plus regular subscription renewal costs. Stand-alone anti-spyware packages can be purchased online for between US$15 and US$40, and subscription renewals can range from US$10 to US$30 per year.

If you're looking to save money, however, there are some free anti-spyware solutions available - and unlike many free solutions, they're not at all bad. Lavasoft offers a trimmed down version of Ad-Aware, for instance, called Ad-aware SE Personal Edition, for free. Microsoft is also currently allowing users to download  its anti-spyware tool for free. The best known free anti-spyware software, however, is the excellent Spybot: Search and Destroy, which has more features and a better spyware database than many paid solutions.

Many of the paid applications available will have demonstration versions available for download from the vendor's Web site. We recommend always trying before you buy when it comes to anti-spyware. Not all programs are up to scratch.

2. Spyware Removal
Of course the most important feature of an anti-spyware package is its ability to find and remove spyware. How well it does this is heavily dependent on the quality of its detection database and on the scope of detection. The good news is that all of the popular anti-spyware packages have developed quite comprehensive spyware databases, but there is yet no official benchmark for detection rates of anti-spyware packages.

Detection methods are a key quality indicator. An anti-spyware package that supports drive scans, memory scans and registry scans will catch most spyware.There are important differences in the removal techniques of anti-spyware packages. 

Most simply immediately delete the offending registry keys, close any processes in memory and delete all the associated files and directories on the hard disk. Some first try to run the Windows uninstall routine for the spyware package. A good package will use a delete-on-reboot system for files that refuse to be deleted during runtime.

It's a good idea to look for anti-spyware with a quarantine and recovery system. A scan may produce false positives, or, more likely, delete spyware that you find you need. Many applications that are bundled with spyware will perform a check to see if the spyware is still installed, and will not function if you have deleted the spyware from your system. If that's the case, you can either choose to live with the spyware or without the desired software. 

If you choose the former, a recovery system will allow you to restore the spyware to its former glory. (Some anti-spyware software does come with known workarounds for dependency checks, in order to make the software think its associated spyware is still installed when it is not.)

Good anti-spyware also has a whitelist system. If you decide to keep a particular piece of spyware on your system, you don't want it setting off alarms every time you perform an anti-spyware scan. A whitelist system works like a firewall's allow/disallow system - you indicate that you've allowed this spyware, and the anti-spyware will not ask again if you want to delete it.

So, the questions to ask about the anti-spyware removal system include:

• Does it remove all files and directories associated with the spyware?
• Does it remove Windows registry keys?
• Does it remove the spyware from the Windows uninstall list?
• Does it offer a reboot option if it can't delete files?
• Does it offer a restoration feature if I want to re-instate the removed software?
• Can I tell it not to detect a given piece of spyware in the future?
• Can I customise what it scans?