If you simply are looking to reformat your hdd before you reinstall a fresh copy of windows you are better off following Davcon's advice and just reinstall windows without reformatting so that you have your previous files.
A simple analogy for reformatting your hdd is that reformatting is just like taking the labels off of a file cabinet, that's it. And until new files are ready to be written in the same place as the old files, the old files are not erased.
If you actually want to erase the files before you reinstall windows you will need to use something like Killdisk. It will erase the files from your hdd. I am sure there are other programs that will do the same thing but I use Killdisk because it is simple to use and free.
My advice would be to use killdisk and erase you hdd then reinstall windows from scratch. As long as you don't have a manufacturer's disk that is bloated with crapware you will have a new fresh install of win7 that runs like a champ.
Is Your HDD partitioned into a C and D drive?
Have you backed up all of your Files and data?
The following will reformat your drive.
Select "Custom Install"
On page asking where to install - delete all partitions.
When Partitions are deleted, And you are back to page asking where to install. Select your HDD, if you whan more than one partition, just specify how much of the HDD you want for your C drive (Note win 7 will also create a small partition (about 100 MB) in addition to your C)
My issue is similar. I wasn't given an option to create partitions initially like every other version of windows. I started install, quit, and now I have a 100 mb partition that shows up as unallocated (possibly master boot drive) space. I suspect this could be the files from the setup I started, quit, and then began a new install. Is there an option to partition during install? Regardless, I'd like to swipe my HDD completely clean, format, and start again, but Win 7 is new to me and the help menus suck. I know this sounds pretty stupid, but I've rocked vista for a long time and 7 just seems too user friendly for me to find what I'm looking for.
Just select "Custom" install. When you get to the page where it ask where to install, click on the advanced and delete ALL partitions. Then select your now unpartitioned drive to install to. Change the size to what you want for your "C" drive. Win 7 will still create a TINY 100 Mb partition. It storys recovery information there if I remember right. Note: You can, but do not have to partition the remainder of the HDD yet. I normally do not and wait until I've installed win 7 and it's working. Then I use win 7 Disk Management to initialize/partition/format the remaining space