Keep in mind that as time goes on, transistors fail on silicone. A CPU that is 4 or 5 years old will be "tired" because upwards of 10-20% of the transistors have failed and it just doesn't have the processing power it had when new. CPUs are built to have transistors fail, but over time it can really degrade performance. Combine this with mismatched memory, probably some heat issues, an older hard drive that, even if clean, probably also isn't running as well as it should, and a bloated OS like windows 7, and you probably have a machine that doesn't perform all that great. On top of all this, you are probably used to seeing computers run better, so your perception plays a huge role.
All is not lost though. Despite poor "desktop" performance, it doesn't mean it can't run programs or even games. Most games pretty much take over the computer so all the "bloat" going on with the OS in the background is minimized and even a fairly sluggish CPU can run games pretty well. Load times might be slow, and the occasional hitch is expected, but overall once you are in a game it might run better than you imagine. Of course, if you are running the integrated graphics, you should already expect really poor performance, so a halfway decent graphics card, even a "tired" one that has seen it's fair share of abuse, will go a long way toward making it an acceptable gaming rig.
The advice here is good, maximize the memory, keep the file system clean of malware and viruses, and throw a gaming graphics card in it and you will probably be satisfied.