Sam, they slow down because the disk gets disorganised (I think as a result of month after month of security patches); also you may find that lots of things run at startup.
The hardware of the computer doesn't slow down, it's just the OS. This is one of the dirty secrets of the PC world that no-one wants to fix because they wouldn't be able to sell you a new machine every 3 years. Since Mac and Linux do not suffer from this problem, a company with the resources and talent that Microsoft have should be able to find a solution - but instead we get new face-paint.
You can make your computer run like new simply by reinstalling the original OS (not exactly, as it will be handicapped by several years' patches for Windows that will get appled after reinstallation).
More practically, replace the hard disk with an SSD or hybrid drive which make the effect of disk disorganisation much less severe (if you have recovery DVDs you can do the disk swap yourself, unless you have one of the modern machines where the HDD bay does not have an external lid).
As for me, I run a 6 year old Thinkpad with an SSD - and Vista!