First of all, if you have a 10 year old computer with original parts and you're still getting good use from it, that is really impressive.
Generally 4-5 years is about the maximum lifespan of computers these days. Components wear out and break and, more commonly, they just become obsolete and too slow for modern tasks.
As far as your 2-year old computer running slowly, there are a few things you can do, if you haven't already.
- Upgrade the RAM- 2GB is usually enough, but that depends on what you're using the computer to do
- Defragment the hard drive- You can use the disk defragmenting software build into Windows, or there are a few good (and free) tools available online.
- Delete unnecessary temp files that are cluttering your hard drive- Again, there are free tool to help you do this.
- Clean and defragment the registry- Absolutely make a backup of your data (and of the registry) before you do this step.
- Scan for viruses/malware
Of course, there are many more things that can be done, but this should get you started.
Alex @ Jackrabbit.com