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.

Here are a couple options:

A) Hit up a bunch of adult websites and click on every banner ad you see.

B) Go to a torrent search engine (http://thepiratebay.org/ or http://www.torrentz.com/, for example) and do a search for "keygen". Download a bunch and start firing up the executables within.

You will definitely end up with some nasties sooner than later using these techniques.

Very funny how people can get viruses without even trying and then when you want one... its pretty much impossible to find one. I have a bunch saved to a thumb drive to mess around with later and a bunch in the "virus vault" in my antivirus software. A good place to start would be to check the virus vaults in the antivirus software for all the computers you have access to. You can then restore the virus to wherever you want.

Honestly? Start googling for free antivirus software. You are guaranteed to find something on that list, especially if its something you've never heard of. Web pages that scream you are infected! in red capital letters ten feet high (okay, maybe not that urgent) are dead give-aways that you've found badware for the taking.