( use http://www.vmware.com/products/converter/ to create the virtual machine image and use http://www.vmware.com/products/player/ to run the virtual machine image – if you do go for a new PC later, you can take the image with you )

It is unlikely to be a hardware issue (other than perception).

Motherboards when they go tend to cause intermitant faults or complete failure rather than a general slow down. Hard disk start having read problems and you hit sporadic significant slow downs and read/write warnings. Slow-down over time is a well known problem with Windows – avoided in large corporates by very tight lock-down and profile management.

If you want to play the latest games, use a lot of heavy weight applications, or do processor intensive activities (such as photo and video editing) then, yes, you probably need a new PC.

PCs have continued to advance rapidly ove the last couple of years. You will see a huge difference in the performance of the latest hardware compared to your old hardware.

If you do go for a new PC, split the hard disk between OS & Apps, and data. Take a full image of the OS & Apps partition BEFORE you install anything and another after you have installed all of the base applications you want to use. When your machine gets slow again (and it will), you can restore the images you made rather than trying to clean up what you are using.

It does sound to me like you just feel ready for a new PC. It would be worth doing the reinstallation I mentioned and then seeing that whilst performance is much better, the machine is still too slow for the latest applications. (Obviously, it makes little difference for most Internet activities as there you are constrained by the speed of your connection.)