for me
I cant speak for Johnb, but if you are having to run Combofix that often, then something is "funky". Considering the point your at, I would reinstall.
A repair install will "repair" the Os, a better term would be "replace". Any incorrect entries that the OS uses will be replaced. Be it the registry, System files, etc...
You should backup your user files and programs before you do this though. I backup my computer a few times a year (for peace of mind) with a Western Digital external 500GB hard drive. And I do not mean one of the little portable hard drives they also offer, I dont trust them.
I also bought Acronis as the backup program. Though you can just use the backup program that comes with the HDD, if it includes one. It doesnt take more than 45 minutes to run the backup and its a life saver if your computer does die.
You could reinstall the OS, but I believe that in most instances a repair will do just fine. Though if you cant clean the viruses, trojans, etc off before repairing, then the repair will do you no good. A repair install will only help if you are already virus free.
To completely free of malicious items, you would need to reinstall the OS. It sounds bad, I know. But it really isnt all too hard to do. you want to backup your user files and programs before doing this.
A word of advice if you do use a backup. Scan the backup VERY thoroughly and slowly before putting the files back on your computers hard drive.
I have a big question for you, and please take no offense to it. How computer savy are you? Do you have the time and tools to take care of this yourself? If this is more than you feel like you can and/or want to do, then dont feel bad for that. I don't want you to get in the middle of doing something a start to feel overwhelmed. Thats a bad feeling.
(I got that feeling one time when I was halfway through pulling an engine apart, I realized that I didnt have the time, tools, or ability to put it back together! ha ha ha It must have been funny watching me dragging all the parts into the machinist and begging for help. He put it back together for me, and showed me how to do it as he went.)