Alternatively, you can upgrade your computer.
Granted, a PC will not magically stop working after a certain amount of time... but applications will stop being updated, as it can be hard to do so when the OS is not getting all those wonderful improvements that make things easier.
And Microsoft *won't* continue providing security updates (unless... they do anyway?
cf. Internet Explorer vulnerabilities).
If you don't have the money to buy new hardware, there are other options for the mildly adventurous. Why not give linux a try?
Ubuntu is getting quite popular these days.
Personally I favor
Linux Mint (the desktop is much friendlier, and closer to what one would expect coming from Windows).