I honestly don't see what the big tragedy is. You yourself said that it was up for a mere 3 hours. Surely, as Vorlin has suggested, there is nothing of much worth on the machine. At the very worst, it is perhaps being used as a launching point for further attacks. That will be rectified whence you get home, will it not? Not something to cry about, right?

Concerning Vorlin's paranoid claim that your "so-called friend" acted in a slightly unfriendly manner, I think it should be taken with a grain of salt. Some tools might have scanned a block of IPs that you merely happened to be in, as such your computer was perhaps not specifically targetted as you seem to think. It was most likely a coincidence, is all. Also, there are tools that will construct random IPs to scan, quietly compiling a list of random targets.

My question is: why didn't you secure your box before bringing it online? You are the only one to blame. However, your "i will learn from my mistakes" attitude is highly commendable.