I installed windows 8 myself. Ubuntu is also on the same harddrive but i dont use that harddrive at all. Ill fill in my profile now. Thanks
OK, but is this a dual boot system?
Y'know, if it were me, and if I could easily get to either/or Win 8/Win 7 (or even Ubuntu), I'd leave things like they are. If Windows 8 should crash, your files are on another drive and won't be affected.
However, should you decide to change to the Windows 7 drive, there are several things to consider, especially if this is a dual boot system, not the least of which is your boot file. I'll leave that for someone with more experience about boot stuff.
Since you want to clean install Windows 8 over Windows 7; that is the only option you have. You cannot bring an OS from one drive to another. Once you've installed Windows 8 on the Win 7 hard drive, be sure everything works first, and then delete Windows 8 from the secondary drive. If you're not going to use Ubuntu, I would simply format the 150GB hard drive and use it for my data; that way if the unspeakable happens, you will have your files intact.
After installation you may be told you have to re-activate, which should give you options on changing the Product Key or calling Microsoft via phone. Of course, you don't want to change the Product Key, so you would need to call Microsoft and explain that this is the same computer (they'll probably know that) and that you just changed hard drives. Microsoft's licensing says you must have no more than one copy of your licensed product installed, even if it's on the same system, so I would get rid of the old copy of Windows 8 before I called MS.