This thread is all about giving you the best performance possible without the need for you to upgrade your computer.
The advice given here is quite thorough and may seem daunting at first, and granted, it will take at least an hour or two to do everything, however you should see a good improvement in your computer's performance - and not just for games but everything you do with it.

The posts below are placed in a logical order. It's suggested you run through them one at a time in the order they are presented in. You don't have to complete them all, but you should try for the best performance gain.


Part 1. Removing adware / spyware from your machine
This is all about removing popup enducing rubbish from your machine, to free resources (make it faster)


Part 2. Scan Your Hard Drive For Errors
Very occasionally errors can happen on your drive. It's always best to solve
these to prevent future problems.


Part 3. Clear Out Your Temp Files / Freeing Disk Space
Clear out those files that are just taking up space.


Part 4. Upgrading Your Graphics Drivers
Up to date graphics drivers will give you better performance and better quality graphics (so long as your graphics card isn't too old). You can find the how-to-update guide here.


Part 5. Smoother Performance Via A New User Account
Creating a new user account is a great way to hopefully speed the game up. At the very least, it will make more memory available to the game, which is always good.


Part 6. Clean The CD/DVD
The cleaner the disc, the easier it is to read. The easier it is to read, the faster the game loads when you first run it. You can find the how-to-clean guide here.


Part 7. Defragmenting Your Hard Disk
Defragmenting is a method of organising all your files. End result: Your PC loads apps and games quicker.