NOTE: This only works on Intel CPUs. You can probably find one for AMD if you google it, but as I don't have any, haven't really looked. Only thing I could find was a linux version you have to compile yourself.
Step 1: Get Linpack
Download it and unzip it and save it to a memorable location.
Step 2: Using Linpack
You should now have a command prompt/terminal window open. Press return once so it says "Number of equations to solve". Input a large number like 5000, and hit enter. Then, for the leading dimensions for array, enter a larger number, for example, 10000. Note: you may want to run less equations for older CPUs, otherwise they might take over a minute just to solve one! Next, enter about 200 trials, and then the amount of ram for it to use (in kb). You will want to change this according to how much ram you have free, I gave it 100000, which is just under 1GB. If done correctly, it should then start giving you benchmarks. I have found mine varies quite a bit, but I have managed to get up to 43GFLOPS on Ubuntu with it overclocked to 3.3GHz. Feel free to post your benchmarks in the comments!