Loss of internet access on home network after upgrading to windows 7

I am on a desktop computer that I built. Until recently I ran windows xp. I shared a built in wall Ethernet jack with another desktop. I used my router (Linksys Befsr41) as a switch by disabling dhcp and connecting the wall ethernet and the two computers to the Lan ports on the router. The other desktop until recently was also running windows xp, but was upgraded and is now an apple imac running os x lion. My internet briefly stopped working but I was able to reset my router and get my internet running again. I then needed to reformat my build and installed windows 7 ultimate 32 bit. I cant quite pinpoint my problem. If I connect straight to the wall ethernet jack i have internet access. However if i use my router as a switch to get internet for both my pc and the apple imac, I will not have internet access. Sometimes the internet does indeed come back on in this circumstance, only to get disconnected again shortly. I am not sure who to blame because the router worked fine when spliiting ethernet for xp machines, and I have internet access when running straight from the wall to my pc. I tried updating the firmware of my router, however the problem persists. thanks for the help.

I reinstalled windows 7. If i power cycle the router, then use the command
ipconfig /flushdns follow by ipconfig /registerdns
i will regain internet connectivity. However after any restart of the computer i lose internet connectivity and must follow the above steps to regain it.