There are a number of things you can do but some of these depend on what your computer is and whether it can be upgraded.
1. Add more memory if possible - this will help in most cases. Use the System Scanner on the Crucial memory web site
to help you understand what memory can be added to your system.
2. Clean up your hard disk using a tool such as CCleaner
to delete lots of unneeded temporary files, internet cache etc.
3. Reduce the number of programs that start up with Windows - you can again use CCleaner to do this - open the Tools section then click on Startup to see a list of what is being started each time you start Windows - there are normally a lot of extras that can safely be 'disabled' and which will speed up the system startup.
4. Defragment your hard disk (after doing the clean up above) so that you make your files more contiguous rather than being scattered all around the disk in fragments. Use one of the free third-party tools to do this such as Iobit's Smart Defrag
or Piriform's Defraggler
- you can also use the default defragment tool in Windows.
5. Reduce the size of your Internet Browser cache - you probably don't need more than 50 - 100MB of cache - anything more is just going to add to the defragmentation of your disk, and require time to search through when comparing what is held in the cache with the pages being loaded in the browser.
6. Lastly, if your computer hardware is very old then you will probably have reached the limit of performance when running Windows but you could get better performance using one of the smaller Linux distributions such as Puppy Linux
which works well with older hardware and can be run from a CD to get an idea what improvement you would see.