some time to wait for computer programs to work (if your computer is long due for a cleaning, might have to be ready to let these run all night)
internet access
Step1
If (a) your computer is working so badly that you can't even download programs; or (b) you don't have system restore or don't have a restore point, and/or can't restore your system to a time when you last remembered it was working well (or can't remember this restore point), first, run scan disk and disk defragmenter in system tools (which you will find by clicking on start, then choose "programs", then choose "windows accessories", and choose "system tools"). You will find each of these programs in the menu there.
Step2
There may be better free registry cleaners and disk defragmenters online, but I've had success with these, and I don't trust most of the others. I would download (from download.com or www.auslogics.com website) the latest free versions of disk defrag and registry defrag by auslogics. Run both of these (the order of use depends on how messed up your computer is, at the time; registry defrag will require you to turn off your computer, and you can't run both at once). Disk defrag might take a long time! Regarding disk defrag: you may be shocked at how much better this is than the disk defrag in windows. You also may want to run it twice, if you have over 1000 fragmented files that are defragmented the first time - the second time will go a lot quicker. Also, download the latest free version of ad aware (this is not to be used in place of, but is in addition to, your antivirus program, which you must leave on your computer!), and run this. (The free version of spyware doctor is also quite good, and will always find more spyware than ad aware, but my experience is that it slows up my system more, afterward; you may wish to download this, and disable it most of the time, then enable it once a week, download updates, and let it run on this type of schedule.) Please remember that after adware or spydoctor has completed the scan(s), you must follow the instructions to remove the adware, as the adware located is not automatically removed; you cannot just close the program when the scan is completed and the report of what needs to be done is on your screen.
Step3
I then use windows live safety scanner, which you can only use with internet explorer (what a surprise - Microsoft won't let you run this with Mozilla or any other internet browser). It's quite effective and trustworthy. You can find this at http://onecare.live.com/site/en-us/default.htm. Let it run all the way through - this step, too, especially the first time, may take all night. Please remember that after the safety scanner is completed, you must follow the instructions and take the extra steps needed to clean your machine; you cannot just close it when the scan is completed and the report of what needs to be done is on your screen. You may wish to run registry defrag one more time at this point.
Tips & Warnings
I generally clean my computer, using ALL of the above, after doing a virus scan, at least once a week.
I have downloaded, and am using, windows live one care 2.0 as my antivirus program (the safety scanner is somewhat better for your "once a week computer cleanup" since it also cleans your registry). You'll find a review for the beta version of 2.0 which was being tested until early December (when the final version was released) at http://www.pcmag.com/products/0%2C%2Ctqs%3D8F4134C2BCC272A5696CED71CE218FA87408A041%2C00.asp?query=windows%20live%20one%20care&action=newsimplequery&cid=1562&sid=1562&gridtitle=Search+Results&stpdinglp=1.
A word of caution: if you have a different antivirus program, and you're a little queasy about putting all of your trust in microsoft due to the prevalence of hackers who go after microsoft, or the vulnerabilities which have been found in microsoft, you may have to delete the other antivirus program(s) to use onecare, unless you are using mcafee (windows must have a side deal with mcafee, since it requires every other spyware system I had or tried to be removed, but mcafee).
I am not a computer professional, and do not claim to be one. I will say, however, that the above was successful in cleaning my computer, when several pros had told me my computer was shot, and I should give up on my computer, download whatever information that could be saved, and get a new hard drive or computer.
This will not work to restore your system if you have a virus on your system.
Remember to make a backup restore disk, if you are not backing up your files!!!!!
Novice 
