When people ask to [quote]spend as little as possible[/quote] I usually direct them to Dell. But if your knowledge is low, Dell are not always the best option.
I buy most PC bits and pieces from click here including my current laptop and most of my current PC. Their prices are good and their customer service excellent.
You will need to buy a PC with Windows - preferably XP - but otherwise, no need to buy any software. You can get excellent office applications (openoffice), image manipulation (serif, irfanview), CD burning (deepburner) software free of charge as well as all the security softare you need.