PC upgrade: Hardware specifications
It may seem obvious, but the very first thing you need to do is to check your existing hardware spec, then research what new components you'll need then decide how much you are prepared to spend.
Cliff Saran provides a step-by-step guide to updating an ancient Pentium 4 3.2 GHz XP-based machine with 2 GBytes of memory to an AMD Phenom Quad Core PC with 4 Gbytes of memory running Windows Vista.
The hardware is built around a Gigabyte GA-MA790FX-DS5 motherboard.
NB: PC components can easily be damaged by static electricity - I recommend you use an anti-static wristband.