I use AVG, Ad-Aware and Spybot Search and Destroy. It is always good to have a mixture of programs because while none are perfect, you can keep your system clean with a few. The ones I use are all free.
http://www.download.com/3640-2239_4-1078...
http://www.download.com/3000-8022_4-1076...
http://www.spybot.com/en/index.html
If you don’t want to download, try running some of these online scans. Online scans allow you the privilege of cleaning your system when a virus will not let you run you protection. Find them here:
http://www.infectedornot.com/usa/
http://www.nanoscan.com/as/index/
http://www.virustotal.com/
http://www.cyberdefender.com/
http://housecall.trendmicro.com/
http://www.garbageclean.com/
http://secunia.com/software_inspector/
Some people have good results with CCleaner. CCleaner is a free program to benefit your system and increase your privacy. It removes unused files from your system - allowing your computer to run faster and get more space on your drive. The program can also clear system history. This program contains no spyware or viruses and is a safe download. Find it here:
http://www.ccleaner.com/
I advise you not to run multiple anti-viruses as they could work against each other.
Anti-virus programs detect and delete viruses on your system. Always make sure to update your computer with the latest program update before running. Here are some other anti-viruses:
http://www.pcworld.com/downloads/file/fi...
http://www.pcworld.com/downloads/file/fi...
http://www.pcworld.com/downloads/file/fi...
http://www.threatfire.com/
Here are some firewall programs. Firewall programs are designed to protect you computer from unauthorized and unwanted communication through your network. Again, don’t run more than one and remember to always make sure you have the latest update:
http://www.pcworld.com/downloads/file/fi...
http://www.jetico.com/
One for Vista:
http://www.pcworld.com/downloads/file/fi...
Here are some anti-rootkit programs. Rootkits are malware that can take over your computer:
http://www.pcworld.com/downloads/file/fi...
http://www.pcmag.com/article2/0,1895,211...
http://www.pcworld.com/downloads/file/fi...
http://www.pcworld.com/downloads/file/fi...
http://www.pcworld.com/downloads/file/fi...
Finally, some spyware removals. Spyware also allows people to control and learn about you by gathering information from your computer.:
http://www.pctools.com/spyware-doctor/
http://www.pcworld.com/downloads/file/fi...
http://filehippo.com/download_spyware_te...
An article about spyware removal:
http://www.pcworld.com/article/id,136205...
And an article about good security programs:
http://www.pcworld.com/article/id,133631...
Don’t forget that your computer comes with tools to keep it clean. When files are saved to a hard drive they are broken up into tinier files. That file is called a fragmented file. The fragmented file takes more time for the computer to put together. When a file is defragmented, the computer takes all the bits of files and puts them into one place which makes for a faster computer. Defragmenter does the process. To find Disk Defragmenter, go to Start-All Programs-Accessories-System Tools-Disk Defragmenter. As well as Disk Defragmenter, there is something called Disk Cleanup. Run it as frequently as Disk Defragmenter, about once a month to keep your computer healthy.
Keep in mind that if the problem is serious, you should boot your computer up in Safe Mode. To do so, hold down F8 at startup.
I hope this helps!