How to delete the other Windows Operating System when I have two

About dirty Windows folder:

a) Simplest way: Browse C:\Windows, hit the F3, search for everything. When searching complete, select all items and delete them, skip protected items. Then move C:\Windows under the carpet. Now you freed the space. If you really want to delete the remained files; take their ownership, then delete them again.

b) If you have Windows 7 Install disk, boot the PC from install DVD. Format the drive C:, go back and repair your Windows 7 -because probably it won't boot. Or it will.

c) Boot form Windows 7 Install disk. Open a command prompt. Type del c:\windows /s or Format c:

P.S: If you don't have the DVD, Win 7 has embedded recovery tools, just turn the power down, next boot, it will ask for recovery.

About boot menu:

Right click My Computer > choose Properties. At the left side click Advanced System Settings link. Select Advanced tab. There is a Startup & Recovery button. You can choose the default OS and menu duration. If you set 0 as duration, menu won't appear next time. If you want to totally rip the older OS menu item, there are a few tools that modify startup menu.