If it's a public IP address you can do a whois on it in order to find out the ISP that owns the IP. After obtaining this info you need to contact the ISP, however, you will probably need to provide a subpoena if you want to find out the person that used the IP at the specified date/time that the person logged onto your work computer.

To add to the above, you will likely not be able to get a subpoena without involvement of your legal counsel (or law enforcement).  In my experience, law enforcement will not get involved unless there are threats of bodily injury or you can demonstrate substantial financial loss.

If you do not pursue the criminal aspect, there are still civil remedies for losses suffered.  You will need your legal counsel to get a subpoena then (through the courts).  You have a limited time to act as most ISP's only keep IP/username correlations information for 30 days.

Good luck.  This is not something that is easily done by non-professionals.  You have a tough road ahead.

Not to mention if he/she hopped through proxies...