How do you wipe a computer?

I've had to reinstall the OS on my computer several times, because of trojans. Probably 4 times total since I've had it. It is an e-machines I purchased in 2006. It currently runs Windows Vista and I do have the disc with it. However, the computer has backlogged the data over and over again and I have no memory left starting from scratch. Chat logs from years ago are still on there. How do I COMPLETELY wipe my system and start over?