Sounds like it needs some maintenance done, not sure what regime you use to remove junk etc, but try the following and it should help a fair bit, but as you are probably \aware, you could seriously benefit from more ram, so the biggest difference you can make is to upgrade the ram, how do you feel about that ?
Right the longer a system runs without fresh reinstalls the more it gets clogged up, mainly with remnants of unused programs, now you didn't say what size hard drive you have, but open you're "My Computer" and right click on the drive, select properties, then see how much free space you have, if the drive is too full this can seriously slow the system down, so make sure it's only about 60 to 70% full tops for optimal performance, and uninstall anything you no longer use, I suggest uninstalling using Revo, it removes the remnants as well.
Next remove junk files, I suggest using the following tools, the reason for using more than 1 tool is simple, each get bits the others miss, so by using all the following it will remove the most amount of junk, 1 thing to note, if you use cookies to stay logged into sites such as this one, then these cleaners will remove those, but I can instruct you on how to save such cookies if needed, just holla if you want that, but here are the programs I suggest:-
Disk Cleaner
CCleaner; Note - Watch out for the check boxes during the install (true of ALL programs), uncheck the bottom option, unless you want the Yahoo toolbar that is.
CleanUp; XP Only
ATC; The download link is not obvious, so look for the name in red text in the first line of the description, that is the link to click.
The last thing that should help is to run a defrag, I suggest using
Auslogics Disk Defrag, I further suggest coming offline prior too running this, then right click on each icon in turn in the task bar (by the system clock), and select exit\pause etc, those that wont close leave, the best time to do this is when you have finished using the system for the day, then let the scan run overnight, if it is as slow as you say, this could take quite a while the first one or two runs, once the system speed improves you can run it normally, but best results are achieved by not touching anything while it is running.
See if this helps, and once things pick up we can check for possible infection issues if there is any further indications of other things going on, hope this helps.
PS, It took about five minutes to type the last paragraph.