Reformatting your hard drive should'nt slow down your comp at all and in fact, should make it run faster. Reformatting is reloading the operating system, is that what your actually doing or defragging?
I usually reformat my machines about once a year. It gives them a clean slate to start fresh with and it will generally remove any traces of spyware and viruses that might remain.
The common problem for slowdowns are the gradual increase of startups when new programs are installed as well as system temps that have increased over a period of time. Defragging the HD along with the occasional use of a good system registry cleaner to remove now invalid registry velues leftover from removing softwares(called orphans) can help improve overall performance and see faster access times as well as reduction of startups. The use of the msconfig utility to disable non essential items that start along with Windows can free up available memory. The only time a drive should be completely wiped is when it becomes infected with a virus/spyware that has widely spread across the drive itself.