Hi
My two-penneth. (win 98, XP or Vista)
I don't think the anti-virus software is primarily for a faster / slower PC it's for detecting virus's, so any claims to speed up your PC would be secondary.
There are quite a few reasons for a slower speed.
1. Junk mail, like cookies, temp files and other files that you pick up from the internet. There are three ways (I know of) to clear them,
a) Go out and buy a software package. (COSTs!)
b) Run the 'disc clean up' utility - check all boxes and run.
c) Go to 'Control Panel' Click 'Internet Options' click browsing history. This will allow you to delete 'temp internet files, cookies, history (sites you have visited) and form data (saved data you have typed in)
2. Disc defragmentation - the data stored on your hard disc is recorded on it randomly and parts, of one program saved to it, may be stored in several locations 'disc de frag' sorts all the files and copies them to a specific location, so it takes the "reader arm" less time to travel and "see" the program you are using.
(I use Vista; 98 and XP have the equivalent programs, I regularly run disc clean and defrag (set to Wednesday 3:00am - I never turn my PC off))
3. How many programs does your computer boot up at start, eg. the ones visible in the right hand side lower tool bar - (Office Suite, Norton, Mcafee etc.) - these are the ones you see momentarily, at boot up, the more of these running at one time will slow down your PC, as they are still running in the 'background' when you do your own stuff. As time goes on we add more and more stuff to the PC, without upgrading the "capacity" the PC will take.
4. Hard disc space - "generally" the less free space on your hard drive the slower your PC will run.
5. RAM - random access memory - this allows the hard disc to transfer a certain amount of data to be seen / used on the screen - eg. a video clip - low RAM slow choppy image - high RAM - fast streamlined images.
6. Processor speed - calculates how many "pieces of data" can be handled at one time. Bigger the better.
I hope some of this makes sense - I got a 'c' in English but can't put a sentence together to save my loaf'
To summarise
1. & 2. Regularly run whichever 'disc clean, internet options, defrag, installed software'
3. Close any programs, that you do not need EXCEPT *NEVER CLOSE ANY ANTI VIRUS SOFTWARE AT ANY TIME *
4. Remove any 'old games' etc. to free disc space. Bigger hard disc - costly.
5. More memory (RAM) - costly
6. Faster processor - costly and probably not worth it - better to buy a new higher spec PC base.
There are an infinite number of solutions to your question, try the ones that cost nil first, with each version of windows that come out things tend to get 'better' but they all have inbuilt software to make things better, but PC's are out of date within 6 months - with advances the software gets bigger and more complicated - so an older PC can't cope with updated software - so it slows down or stops.
regards
Gary :)