first we should make a list of free antispyware products that are available. Surprisingly, the choice is quite limited. If we omit security bundles that include antivirus engines and trial editions of software (which expire usually after 30 days of use), there we have pretty small number of programs.
SpyBot Search & Destroy comes first as most well-known free application. Then, we have Ewido Anti-spyware which ceased to provide support or licensing options, but still provides definition updates to users of the free edition. Then, a-squared anti-malware offers a free version which deals with spyware infections as well. SUPERAntiSpyware has a free version as well. Malwarebyte's anti-malware finishes this short list.
However, there are important points to be considered. So, most of the above named antispyware products lack real-time protection which is crucial to complete PC security. Second, the majority of the allegedly "free antispyware" programs do not allow the removal of detected infections. They either work as demos or require fully licensed versions. Third, do not forget about the latest threat spreading on the Web like forest fire: fake antispyware or rogue security programs. There are literally tons of them released by cyber-criminals to suck the cash out of your bank account. I will not name a single example here, but you can easily find plenty of names if you do a bit of research yourself.
With all that said, here's a short list of features that you want your antispyware to have by all means. If you have a program already installed and you like the way it protects your computer then you can skip the beneath part or consult it as a basic reference. Those of you who don't have yet antispyware installed or still worse have no clue about what it is, can copy-paste these obligatory features for future reference. You'll find them helpful when making a choice.
1. Updating definitions. Yes, best antispyware provides frequent updates, preferably daily or even several times a day. Signature updates should be incremental meaning that only tiny portion of bytes is downloaded to keep your copy of antispyware up-to-date. Those programs that download big files are eating up your bandwidth. Also, if you happen to use your computer in a non-networked environment without web connection, it's wise to choose an antispyware which offers offline updates. This is when you can download a separate file from the manufacturer's website and apply it to the program when convenient.
2. Usage of system resources. Nowadays that's a common pitfall of many software programs. I guess modern programmers believe that computer owners have a bunch of spare gigs of memory to waste. Well, I do care about every CPU percent and megabyte of RAM taken by running applications. If antispyware in idle mode pikes to 100% CPU usage then I'd stay away from it. Nobody wants to have a protected PC that is all busy with its protection.
3. Detection rates. This is important, sure, but please don't get paranoid about it when looking at various comparison charts. If antispyware A spots a 99.5% detection rate against 99.6% shown by program B, this does not mean that A is worse than B. It's just a result of one particular testing environment. Keep in mind that many antispyware programs actually overlap with antivirus software when it comes to detecting infections. Those comparative tests usually serve for informative purpose only. Don't allow figures really make up your mind.
4. Certification. This is arguable but it's always good if the antispyware of your choice has ICSA certificate. Don't bother though if a program doesn't have it. Uncertified antispyware may actually perform better than a certified one. The thing is that certification process takes a lot of time and newer software hasn't yet managed to pass it.
5. Report no false positives. This is a tough requirement actually, and dubious, too. High detection rate sometimes goes side-by-side with an overwhelming number of false positives. This makes the task of choosing a best antispyware even harder.
Inspired Source: http://goo.gl/0m6dx