How can I get my MacBook to stop freezing after I switch back to Mac OS from Windows 7 in Bootcamp I have a problem I could not find anywhere else. Maybe somebody here has an idea on how to solve it.

I have a MacBook Pro, early 2011, 2.2 GHz i7 with the AMD Radeon HD 6750M and 1680 x 1050 display. I have installed Windows 7 on a partition of the hard drive and have to switch there about once a day. Every single time I switch back to Mac OS X (10.7.2), the computer will freeze either immediately or within up to three minutes after logging in. This is absolutely reliable. I then have to do a hard restart (keeping the on/off button pressed for several seconds), and when the computer comes back, everything works fine until the next time I switched to the Windows partition.

Since this is a freeze, no error report is generated by the system, so the Apple solution is "Send the computer in", something that is impossible for me (I need my MacBook constantly).

I do have quite a few extensions installed, and suspected Quicksilver for a while, but deactivating it did not solve the problem. Other extensions going up at startup are: iCal, Evernote, DoublePane, iTunesHelper, Autograph, DropBox, SophosUIServer. Sophos Anti-Virus is the only program running both in Windows and Mac OS. All software is legal, and virus scans (Sophos and MacScan) showed no Malware.

Any helpful suggestions, what might cause the problem?
My Mac (OS X Mavericks 10.9.4) also crashes/freezes and becomes partly or wholly unresponsive (beach ball and all) and it was due to Sophos Anti Virus. Uninstall it and see if it makes things work!