Download Virus/Malware Purposefully

I ended up finding my way over to VX Heavens, where they have a massive collection of virus broken down in to various categories (Backdoors, DoS, Worms, Hoaxes, Trojans, the works). Great if you need to analyze actual malicious code in a "dead" state (as much fun as surfing to a bunch of sketchy sites is and sorting through the mess later).

If you really want...

You can probably find some sort of virus "history" through standard google searches. I'm sure there are archived copies of such "I love you" worms.

If you are trying to purposefully infect a computer, download Windows XP (no service patches) and using IE 6/7 browse various sites of questionable content.

Then install LimeWire. Search for songs or music. Download as many as you can. Sort them out as real music files and .exe files pretending to be music files (often hidden in archives). Now run them all.

Install just about every browser toolbar you can find for IE, along with any available programs along the lines of "I sit in your taskbar and randomly popup telling you something".

However I feel I must warn you...

Doing the above will most certainly trash the machine. Also you probably will have limited insight as to what actually caused the most damage. You will most surely have various viruses and malware present, but finding and identifying them will be a challenge in and of itself, not to mention dissecting the programs.