I am assuming that your computer has one hard drive which is not divided into two partitions and that your existing OS, your programs and all the data are stored there.
Before you do a clean instal of Windows 7 - which will wipe your disc clean - you will need to copy - or back-up all your data which you wish to retain (documents, music, pictures, videos, Emails etc) onto a sperate hard drive or partiton.
If you do not have a seperate hard drive - and if your one and only drive has enough free space - you could create a second partition on that drive to which you would copy all your data.
After you do the clean install of Windows 7 you would then have to install all your software programs again (you cannot just copy them from one disc to another) and copy your data back to the relevent folders on the C: drive.
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