You can use Task Manager to see what's going on - Process Explorer (free) does much the same thing but with prettier graphics. Either will tell you which process or application may be eating the machine's resources.
You may consider having the machine go back in time to a previous good software state by using System Restore - return to a restore point.
You will do well to access the machine's Startup list and uncheck everything - over time all kinds of things get themselves placed in this list and either load or part-load at start time - some good and some bad and mostly unnecessary except for security software.
You might consider using Piriform's CCleaner (free) to tidy up the machine's registry.
It's a good idea to defragment the disk from time to time - once a year say.
Spybot Search & Destroy (free) will scan the machine for malware - and pobably find quite a few nasties - have it fix the problems found - run it weekly. Spybot also gives you a warning if any site you visit attempts to alter something important in the machine - you have the choice to permit or deny the change.
I do / use all of these techniques and my machine runs OK - a recent and annoying slow-down had something to do with a Microsoft process "svhost.exe" consuming 100% CPU most of the time - a system restore fixed it for me.
Internet Explorer is a fine Browser but prone to external attack - many people are using Firefox (free) - also prone to attack but less so.
Some people may blindly advise you to "Add More Memory" but as the machine probably ran fast enough when it was new it's unlikely you need more RAM now.
Good luck...
Peccavi
February 2010