I've just found another case like yours in sevenforums.com. According to their topic (Menus Open to the Left or Right Side - Windows 7 Help Forums), open the Windows Registry Editor (regedit) and follow the path:
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows
Create a new String value named MenuDropAlignment and set the value to 0. Then close Registry Editor and logoff then logon to see if it changes .
If the value MenuDropAlignment has already existed and had a value of 1, simply change its value to 0 or delete it, then logoff and logon. I've tested on my system, it works well .
It seems like somehow one of the updates must have created MenuDropAlignment with a value of 1.
edit: Oh and I found an option in Tablet PC Settings under Other called Handedness that reads "Change where menus appear on the screen by indicating which hand you write with." And it has Right-handed/Left-handed with "Menus appear to the left of your hand." (or right)
I thought it was the Windows update but it's possible that it was hooking in my Huion H610 tablet that did it.