Originally Posted by xtremeIX
Is this a wireless setup? If so, is the router secured to prevent anyone close enough (like in a car on the street in front of your house or sometimes even a neighbor if the signal is powerful enough) from accessing it and making unauthorized changes? The system should tell you if the connection is secure or unsecure. Be sure to secure it if it isn't already.
Is Windows Firewall up and running on all PCs? Have you examined the exceptions being permitted to remove any you don't use or really need - thus closing down potential entry points into your systems? Please take a look and pare it down to only the essentials you require.
If the problem occurs again, then I'd have to say you may be infected (or hacked) despite nothing detected by MSE or MBAM and would then try a few other free programs to check if anything is discovered - uninstalling the current program to use the new one each time since you don't want more than one AV program (this does not apply to AM or AS program) running on your system at the same time or it can cause comflicts or freezes or other problems. Use removal tools when appropriate http://www.raymond.cc/blog/archives/2009/05/05/comprehensive-list-of-uninstallers-or-removal-tools-for-antivirus-software/. You may also want to try the free Avira at: http://www.free-av.com/ and Avast at: http://www.avast.com/index. Reboot after completing all the scans. You may also want to try the free OneCare at http://onecare.live.com/site/en-us/center/whatsnew.htm and let it run all the options (except the registry cleaner) because that’s good maintenance (it will take some time to complete but can be done in the background).
If nothing is still detected, I would post the situation in our Security Forum and let our security expert, Jacee, take a look using programs too sophisticated for most others (myself included) to confirm that you really are clean as changes like this that "magically" happen for no apparent reason are very often caused by some type of malware. While that's no guarantee (it could be something else though I honestly can't think of anything other than the security issue raised above), at least it rules it out as a cause and then we have the challenge of finding out what else might be causing this.
I hope this helps.