I operate two Dell Dimension CA 2005 vintage. One with Windows XP for Tax and other software which won't run under Wine on the Ubuntu 12.04. I have both running at the same time and use a switch to go from one to the other. The Ubuntu PC boots in about 40 sec and shuts down in 10. The XP PC boots in about 2-3 min. My advice to anyone considering replacing Win 7 with Ubuntu is to remove the HD and store it away. Then put a small 60 - 80 GB disk in the PC and install Ubuntu. Or if you must clone the Win 7 disk and put the copy away. Then install Ubuntu as a dual boot system. I haven't tried this but it should work. If you don't like the Ubuntu Unity interface, Linux Mint is like Windows XP and it is based on Ubuntu.
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