What happens with windows is programs install themselves into your start up routine.
After a while this means things start to slow down because you're waiting for these to be up and running before the computer allows you to use it.
One way to fix this, is to goto your start button, select run, and type 'msconfig' in the box before clicking ok.
The last tab in the window that's displayed is called startup, and inside you will see a list of all programs that start when you turn the computer on. Try unticking as many of those as you can, however, be careful not to disable any virus or security software, nor disable any essential windows components. If you're not sure, you're safest to leave it starting.
Another cause of slow running computers is a corrupt or bloated registry. The windows registry holds information about every program you've installed, or have installed in the past and it's settings. Try downloading one of the many registry optimising programs available to fix this.
Disk Drive fragmentation is another issue. Think of the surface of your hard drive like a giant warehouse. Think of your files as pallettes, and pieces of your file are boxes stacked on this pallette. Over time these boxes can get moved around, and before you know it, one box is at the opposite end of the warehouse to the other, so everytime your computer runs the program, it's running backwards and forwards around the warehouse to collect the information it needs to run.
You can defragment your hardrive by ging to start -> All Programs -> Accessories -> System Tools -> Disk Defragmenter and running the program. This reorganises the warehouse, and puts everything in order. Be warned, it can take a long long time to run.
Ultimately, you can reformat the hard drive and reinstall windows. This is a real pain, but I've found it to be neccessary on a heavily used windows machine about every 2 to 3 years.