If you have insufficient time to do preventative maintenance and spend most of your time solving problems your entire methodology needs to be revised. Rather than tell you what you should be doing each period I'll give you some ideas so that you won't have to do things.
Fist up you need a good monitoring system and as much automation as you can manage. These two items should free up more time than many admins realise until after they have it set up well.
Just a few of the things your monitoring system should be doing for you are:
- Alert you when mail or spam filter queues grow too large or too suddenly.
- Alert you when drive space gets too low, CPU use gets too high, etc.
- Record things like disk utilisation so that you can see trends over time.
- Same thing with mailboxes.
- Alert you when the firewall registers an abnormal number of hits.
- Same thing for anything serving the outside world. e.g. DNS and web servers.
- Alert you if AV updates are too old of if any machine has the AV software turned off or uninstalled.
Defragging shouldn't even be on your list of tasks because it should be an automated process. At your desired interval have the server run disk checks and a defrag after a reboot. Consider tying this in with a system to install queued updates and patches (which have previously been tested on a non-production machine).
Temporary folders can also be cleaned with automation. I create a simple application that is triggered after a reboot which waits for 10 minutes and then cleans out all temporary locations. The delay is to ensure it doesn't delete files that may be required for an install or upgrade that completes after the reboot (learned that the hard way!).
One thing you must do manually at whatever time period suits you is to monitor the monitoring system and automation, just to be safe. I check mine daily but haven't actually encountered an issue for over a year.
When you do get your system and automation going make sure you also have a version control system to put it in. It can be real annoying to discover that last little tweak broke something else but you can't remember exactly what you changed.