Assuming you are using a Windows based machine, here is exactly what I would do if you brought it to me.
1) How much memory do you have installed in your system? CLICK START-->Control Panel-->System. In the lower right you should find the amount of RAM. This number should be MORE than 512MB... If it is not, get more!
2) Check to see if you have an anti-virus already installed. If you do, remove it (Norton, McAfee, ETC.) These AVs cost money and are bloated as to the amount of memory and system resources they take up. Download AVG from the free software links on this blog and install it and update it.
3) Now remove any outdated Anti-Spyware software (Webroot, CA, etc.) for the same reasons I told you to remove the Anti-Virus. Download Spybot and run it. Be sure NOT to run TEATIMER as it is a bit of a resource hog and another program we will install later will handle the intercepts.
4) Download ThreatFire and install it. This is a good antispy/antimalware/antivirus system that uses a new way of detecting and catching threats. It works WITH your basic Antivirus software so you will still need to have one installed (like AVG).
5) (OPTIONAL) Install Comodo Firewall. This is a much better Firewall than the one built into windows but it may be unnessisary if you already have a firewall/router installed behind your cable modem/DSL modem.
In most cases these few short steps will have you fixed up but there are times when you will need professional help. Your local "Chop Shops" (small computer stores that build systems) can help. I am not a big fan of Geek Squad or Fire Dog at this time due to the level of knowledge of their technicians. This opinion is subject to change in the future but for now stick to the small shops.
All of the links to the software I told you to get are located to the RIGHT of my blogs on this site. Bookmark this site so that you can get access to direct links to updates.