Can I use bootcamp without reformatting my hard drive?
Boot Camp doesn't require you to reformat your drive. HFS+ partitions can be live-resized by the Boot Camp Assistant or the Disk Utility without going through the trouble of erasing everything. No file on your Mac OS partition will be affected.

There is, however, the case where you've had your computer for long and Mac OS has written files all across your drive. In this case, you will need to defragment your drive, in order to put all your files in a nice contiguous space on your hard drive, so that all the remaining space is contiguous too and a partition can be made there. As far as I know, iDefrag can do this for about $15. (Disclaimer: I never did it and can't guarantee it will work. But it should. Can someone confirm?)

The other way, if you have an external drive, is to copy all your files to this external drive using a software like Carbon Copy Cloner (free), reformat your drive, then copy them all back to your drive. Everything should be like before and you'll have no hassle except the time required to copy back and forth (your programs and your serial numbers and the such will be intact).