I found it quite strange of all the threads that have been started about this topic, that nobody has written an FAQ on the subject, so I will try and put one together.
Please note: I take absolutely no credit for this as these ideas are suggestions from people who have much more knowledge than I. I will make a reference when I use somebody elses idea.
1. First thing to do is check out this website, it has a full list of programs that start up automatically when Windows XP boots and then tells you, depending on the type of machine you want, which are safe to turn off and what other programs rely on these services. I printed it off and made the changes to my computer and it was great.
bcastner also has an FAQ that deals with this problem, I just ran it on my computer and I cannot begin to describe the difference it makes on booting up time. My computer went from booting in about 45 seconds to booting in 20 seconds, let me tell you I was thoroughly amazed at the difference.
The last two will check your computer for spyware and adware, these are various things that find their way onto your hard drive from websites you viste that give you messages and can slow your computer down alot.
4. Another thing to do is search your computer for temporary files and folders and delete them. Go to search and look in all files and folder and search for *.tmp files. Also in IE under tools --> internet options there is a button you can push to delete all those wonderful cookies that accumulate when going to different websites.
5. One thing to note is that if you have anti-virus software and firewalls such as Norton, Mcfee, etc., these programs are running continuosly in the background and can contribute to slowing down your machine, but as long as you have a decent amount of RAM (most recommend no less than 256 Mb as the min, 512 Mb is just great), this shouldn't be much of an issue.
6. The issue of virtual memory has come up many times before. I won't go into details explaining what virtual memory is, it is basically using your hard drive to simulate RAM for your computer, a more detailed definition can be found here.
There has been much debate on whether increasing virtual memory really increases the speed of your computer, personally I think it does and so I will tell you how to change it and you can read this article because it makes a pretty good argument:
The default size for Windows XP virtual memory is 1.5 times the amount of RAM you have, so if you have 256 Mb then VM = 1.5*256 = 384 Mb etc. Here's how to change it.
Control panel -> system -> advanced tag -> settings button under performance -> advanced tag -> go down to virtual memory and click on the button change. From here just click on the drive that you want (the one with you OS on it) and it will give you an initial size (name 1.5*RAM on your computer) and a maximum size, change it to something you want and your off.
7. My final suggestions are pretty standard ones, run disk defragmenter and disk cleanup. To run these go to my computer and right click on the drive that you want (like C: or D:) and look under properites and they are both there. Personally I use Norton to do my cleaning, there Cleansweep program is much better than disk cleanup, but that is just my opinion.
Again, I take no credit for these ideas as they are just merely ones I found from reading through various forms and I would like to thank cdogg, linney, and bcastner because most of the ideas belong to them.