You most likely only need the McAffe starting up. Or if you have a piece of hardware that needs a resident program to work properly (E.g a wireless adapter).
I assume that you have no virus or worm on your computer.
What you can do is start MSConfig. (Start -> Run -> type MSConfig) and check what's starting up in the startup menu. You will probably have a long list. Uncheck everything except what you know you absolutely need to get started every time. The truth is anything that's in the startup list can be started after you are logged in, if needed, but each program likes to put itself in the startup.
Also look into the services tab also and click on the "hide microsoft services" and see what's left. Here also you can probably disable a couple of things but these are usually more relevant to proper working of the system than the startup items.
After you made the changes restart the computer and see whether it speeded up or not.
There is always a chance that McAffee itself slows down the system. I had a client who also had a slow computer and I traced it down to McAffee. It's a hefty program using a lot of memory and CPU - especially in the beginning when it does the initial scans. If your computer is not powerful enough then it will get sloooow.
You can also try unchecking everything in the startup list and the non-microsoft services, and see how it goes. You can always put them back if you find that something is not working properly.
If that doesn't help, or you see that something puts itself back in the startup list then your computer is infected.And one more thing:
do a Scandisk with surface scan. I found out that Windows XP gets extremely sluggish if it tries to read or write to a damaged area on the hard drive. (Right click on the hard drive icon in My Computer -> Properties -> Tools -> Check for errors -> Put checkmark on both places -> reboot; watch the screen because I found that Windows skips the test at first and you need to repeat the above sequence once more -- the surface scan takes a long time)
That should take care of your problem.