A quick search of "HP Pavilion a6636f " turns up NOTHING. I did find a 6636 howeverhttp://h10025.www1.hp.com/ewfrf/wc/document?docname=bph05259&lc=en&cc=us&product=57648#N30
And the specs of that computer would not support Win 7... not enough memory or enough processor. Also, if you COULD load it it would run very poorly because the driver support for the hardware is not there.
That machine was probably meant to run Win 98 or some such, and will never work with a modern OS.