What are possible Symptoms

It is important to realize that some infectiosn do not present symptos at all. In some ways those are the worst, because these generally either invade your privacy or allow others to use your computer for their own (negative) purposes. For this reason, if you have internet access - or even if you have disks or USB keys from other people occasionally attahced to your computer - you should regularly scanyour computer for infextations, even if you have protections in place and take reasonable precautions. Don't stop using your defenses, but do realize that nothing is a complete guarantee.

• Constant Popups. If you get ads popping up on a regualr basis you probably have some spyware. Also, ironically,  most warnings about possible infections are also a sign of infection, unless they are coming from the anti-malware software that you have installed.
• Unexplained hangs or crashes. This is not necessarily due to spyware or viruses. There are many possible causes, and malware is just one of them, but one it's well worth your while to check.
• Unexplained slow downs As with hangs and crashes, there could be many reasons for this. Malware is one possibility, and it's worth checking for it.
• A lot of unexpeced internet activity  - There are legitimate programs that "phone home", but if it seems like you are seeing more than you should based on what the programs should be doing, there is a good chance that you have an infection.

Things To Do

There are a variety of things to do if you suspect a malware infection. Depending on the severity of the problem, and your technical abilities, you may need to get some professional help with some of the suggested steps. Although these steps technically apply to any operating system, they are geared primarily to Windows in its various versions.

• Run scanners that are already on your system - If you have set up your system properly you already have at least one anti-virus and one anti-spyware product, either separately or combined. That is the first thing you should run. This is something that you should be able to do without assistance.
• Use the built in system tools to delete programs that don't belong - Sometimes it is possible to use the system tools to remove malware programs. It's not that common, but if the option exists, it is usually your best bet. But, do scan your system after you finish, as the removal might just be cosmetic. You may need to use on of the scanners or tools discussed to figure out what you need to get rid of.
• Use an on-line scanner to scan your system - Sometimes a malware infection may disable your software, or you arenot completely confident in your software and want a "second opinion". There are several sites that will allow you to scan your computer. Generally speaking, if you have the web address of the site, you should not need help in running the scans.
• Boot with a CD from an anti-malware program to scan - As noted, sometimes the malware will disable the software. If you boot from a CD, whatever infection you have should not be able to run. However, it is unlikley that you will be able to run the scanner from your hard drive. If you use a CD that already has the anti-virus or anti-spyware product set up, you will have the easiest time.
• Use special programs to see what is starting with your computer and what is actually running, or to stop / delete programs that should not be there - There are a variety of programs that can be used for this purpose. For instance "Starup Control Panel" and Autoruns will tell you what starts up when you boot into Windows, and will help you change those settings. These programs will not delete or uninstall programs, but they will generally stop them from running if they show up (not all do), which is a good start. Generally speaking, most people can use these programs on their own. On the other hand, a tool like "HiJack This" provides far more information, but generally needs someone with a fair amount of techincal knowledge to interpret and use properly. CCleaner is easier to use, but somewhat less useful. Some understanding of how the system works is a good idea here, as well. You can find links to these programs at Other Resources
• Reformat your hard drive - This is a last resort, and unless you are technically knowledgeable, it makes sense to get technical help. This makes sense if you are so infected that getting rid of each infection would mean a massive amount of time or creates a risk of damaging your installation anyway.  Also, if you have something like a "rootkit" infection (this type of program hides itself very well, and it may be impossible to be sure that you have gotten totally rid of it without a reformat), you may have no choice about the matter. If at all possible, make sure you have a solid backup first!