the Extension for the Agent was 4.5 but i use only 4.0.1494 so i deinstalled it first and install my extension from the Agent 4.0.1494
The 4.5 Agent Extension works fine with MA 4.0P3, so that wouldn't have been the problem.
It depends on what version of extension it was. According to the KB- you should have at least v 22.214.171.124 of the agent extension for Win 7 and by default the one that came with epo 4.5 RTM which is build 753 is lower than that, so unless that was specifically installed it needs updated.
You should download the extension pack for 4.5 sp1 from the portal area and install those, although v126.96.36.199 or above will still work with that agent version.