Yesterday after IM'ing I received an IM from a random stranger who basically recapped to me the conversation I had only a few minutes before. He proceeded to tell me he worked in computer programing and was a computer hacker that's how he got into my IM conversation log. I know for a fact it was not one of my friends playing a joke because that's just not how they are.I was so creeped out that I just took out my internet cord.

1.) I thought Yahoo stated upon signing up for IM that they did not store chat history, so how did this person get it.
2.)how can I find out if my whole computer system has been hacked? And what do I do if it has?

Find a good virus scanner (I would recommend avast) and set it for a boot time scan. This should find most virus's, make sure to update it before doing the scan though.

Additionally this doesn't sound like your I'm per say has been hacked. More either your system, the system you were communicating with or your connection. Man in the middle attacks are relatively easy with im's. Were you by any chance using wireless at the time, with someone using wireless and low-level protection (wep) it easily possible to hack the wireless and monitor conversions in less than 10 minutes (3.5 minutes is my record while pen testing)

Forgot to add that make sure this wasn't anyone you know or possible a friend of a friend who was present when you were talking to your friend. Most hackers of any skill are not interested in hacking I'm conversations and even if they never contact the person saying "i hacked you" as this makes them to easy to trace, they prefer anonyminity, unless of course they were a script kiddie in which case you don't have much to worry about

Script kiddie: some little pimple faced youth who uses pre-built software that you basically you click hack and it does everything for you requiring no skill or knowledge