Dropbox is great product, I use it on Windows, Linux, Mac Os and iPhone. But on the Windows version (version 0.7.110), there has been a bug for more than a year now. Dropbox seems to consume “something” in Windows that cannot be allocated anymore. The bug appears on Windows XP, Vista, 7 32 and 64 bits.
When it appears, a lot of programs crash, they cannot show any kind of windows, menu or any windows user interface, the only way to save you from rebooting is to quit some programs to be able to launch the task manager and then kill dropbox and restart it again.
I have to say I never shutdown my computer, so it might only happen on computers running for days.