[dba-Tech] wireless invasion

I'm using a wireless router to connect to a second computer. The main computer uses cable internet via a regular modem. 

The second computer is running Vista and almost daily shows activity that isn't mine -- downloads, changes to email preferences -- some weird stuff. My first thought is that someone's using the wireless connection, but I'm not seeing evidence of this on the main computer -- just the one that actually connects via the wireless. 

I'm running the Windows firewall and when I installed the wireless router, I secured it properly. 

I don't know where to begin to troubleshoot this problem. Any help? 

Susan H.