Speeding up the computer:
1. Making sure we have enough "Virtual memory" or decent sized pagefile, to be used as main memory when the RAM isnt enough.
Right click on My Computer>Properties>Advanced>Performance Settings>Advanced>Virtual Memory
You will see that the Operating system drive C is assigned with a specific amount of memory. Now, it would be very helpful if we knew the amount of RAM your system has.
I'll just assume that you have 512MB of ram for now, then you can keep the virtual memory set to anywhere between 1500MB to 2000MB to get decent performance out of it. So, set an initial value and a final value and press 'set' and 'ok'. This should help a little.
2. Deleting temporary files and also some unwanted files:
Go to Run, key in "%temp%" and hit enter, the temporary files folder opens. Just select all the files inside and "Shift+delete" for a permanent deletion.
Also you can find a folder called "Prefetch" in your Windows folder, inside Operating system drive. You can clear the content present inside that too, to clear all unnecessary files.
But both these, wont be affecting the speed to a very high extent, but every bit counts.
3. Disabling things from Startup through MSCONFIG. I feel you can still disable many more programs from there, which will give a boot speed bump. I prefer disabling everything and enabling only the ones which I really need. This gives you some time to analyze what you really need.
4. Disable windows sounds - a great speed improving step. Go to control panel>audio>Sound profile to none. I have given very short steps, but you should be able to do it.
5.
Another very good step is to disable all effects which can drain system performance: Right click "My computer>properties>Advanced>Performance settings>Visual effects>Adjust for Best appearance>Ok
Eventhough this doesnt really speed up the operating system, it gives you a refreshing feel as the Menus tend to be a lot more responsive than before. Trust me on this. If you need specific visual settings enabled, you can do that yourself or just check mark those options in the Visual effects list, once you have unticked all of them.
In all these, I have assumed that you have a windows XP machine. For Vista/7, the steps vary only a little, but do let us know.