Although it's quite well possible that your My Documents will be saved and so will be any data in top-level folders you created yourself. That might depend on your choices when answering the usual questions in a Windows setup also. But I wouldn't reckon on it and be sure that you have multiple copies of what you don't want to lose.
Are you sure your old PC is recent and powerful enough to run Windows 8, and that all hardware is compatible?
Did you run the upgrade advisor for Windows 7 from http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor ?
See http://www.techmynd.com/windows-8-upgrade-advisor-download/ for the Windows 8 version. You can run that as a separate program.
I have an 9 yr old win 32 bit XP media ctr pc with lots of corrupt files Id like to completely wipe the HD clean and install win 8 using the download option I've read about. Am I correct in assuming that it would be like a fresh system (w/o any of my xp programs and files)?