First of all, what antimalware application are you running? Old versions of Norton, McAfee and others are HORRIBLY burdensome on older computers. If you are using one supplied by your ISP, get rid of it. Removal of these, and replacement by something cleaner like Panda Cloud, Avast or Avira might speed up your computer quite noticeably.
If that's not the problem, the first thing is to run a couple of anti-malware programs. You need to scan for viruses and run something like Spybot Search and Destroy to rid you of adware, spyware and whatnot.
The second thing is to check out which programs you're running at start-up. This is not just what's in your Start-Up folder. There are many applications that will provide you with a list of your start-up applications and that allow you to deactivate them. But make sure you know what you're doing here -- you should only make one change at a time unless you KNOW that what you're preventing is not going to affect your system adversely. But basically, get rid of everything you don't actually need.
But probably the best advice is that, well, it's probably time to consider getting a new, more modern computer and retiring your old XP warhorse.
If this doesn't help (or doesn't help enough), first get a disk cleaner like Windows Washer and clean the disk, wiping all the free space. Then get a decent drive defragmentation program (Raxco PerfectDisk is the best I've found) and do the "smartest" defragmentation of the drive that the application will allow. Also defragment the system area.
If this doesn't help (enough), get a decent registry cleaner and clean it up, then use something like NTREGOPT to optimize the registry. Note two things about this: 1. It is dangerous -- back up the registry first and be VERY careful, and; 2. I have never yet seen a PC speed up appreciably by cleaning or optimizing its registry -- it seems to be pretty much a myth.
Now I've got to tell you, a lot of times, even after doing all these things, the computer may still run slow. In this case you may have to bite the bullet and, making sure you've got everything backed up, do a wipe-and-load of the O/S from initial distribution. This is a ROYAL PAIN IN THE REAR END, because, for one thing, you have to update the O/S to current over many, many time-consuming runnings of Windows Update until one FINALLY comes up clean, even before reloading and reconfiguring all your apps, but it gives you a computer that, unless there are hardware issues, is actually better than new.