Most likely scenario is capturing wireless traffic - if you sent any credit card or account information over that WiFi, change all passwords and notify the banks that your cards may have been compromised.
When you turn the computer on, turn off the networking so you do not broadcast a wireless signal.
Go to Finder, Applications, Utilities, and run Activity Report and look at the information showing for CPU, you will see a panel that shows the processes that are running. See if anything in there looks like software that captures keystrokes, spy information, anything suspicious. It is less likely anything was put on the machine, more likely that they were capturing data transmission.