I'm sure Dave will take umbrage, but in my opinion:
The simplest way to stop Windows slowing down over time is to not run Windows. *nix rules. Others in no particular order.
Erase the disk and completely reinstall only those things you know you still want. A painful process but effective.
Never install anything just to try it without taking a full backup first. Uninstallers never remove all the junk. If you must experiment, try it in a cloned VM first.
Fit all the memory your motherboard will support. Note that there is a trade-off in battery life for laptops/notebooks.
Have more disk than you can poke a stick at. Insufficient free disk is asking for fragmentation and slowdowns. A 26,000 fragment page file is not a pretty sight. (How it got like that I don't know. It was allocated with a fixed size on an almost empty disk.)
Install the page defrag utility from Sysinternals
http://technet.microsoft.com/en-us/s.../bb897426.aspx and configure it to run every boot, or at least until it says all the files it wants to tidy up are in 1 extent.