Lack of regulation
Problems for the computer literate:
1) Some of us are still using PCs with 2k installed - which isn't supported by the latest versions of some software (eg Quicktime). Is anyone offering to pay for XP upgrade and extra RAM?
2) The new 'secure' versions of some software are less stable and more bloated than older versions.
3) Newer, more secure versions of some freeware / shareware have fewer features than previous versions (eg dB PowerAmp doesn't have free mp3 re-encoding, but earlier versions do).
4) The only realistic way to keep patched is to allow all those automatic updaters freedom to do what they want (ie install Safari when you're not paying attention). It's too easy to forget to untick the 'Install Yahoo toolbar' etc options on things like Java.
5) Legacy software sometimes needs legacy versions of plug-ins to run properly.
There's no easy way to address these and similar issues - companies like Adobe could be forced to ensure full backwards compatability with obsolete OSes, or made to continue patching up older editions of their software long after they've been replaced by new versions, but this would be impracticable. Plug-ins and OSes could have deadlines built in to turn the software off and so prevent it being insecure - but this would enrage many users.
My suggestion would be for the governments to licence software and require it to meet minimum security specs. Sure, no one can make 100% secure software, but I bet all the big software houses could afford to spend a lot more on ensuring security than they currently do. Medical drugs, food, toys, vehicles etc all have to meet specific safetly regulations - considering how many of us channel our personal details through our computers, shouldn't software with exposure to the web also be regulated. This wouldn't eliminate the problem, but should dramatically cut the need to update every month.