There can be two reasons as to why the install stops at 20% during the install.
The first one is you have errors on the hard disk drive you are trying to install windows to. You are best of doing an error check of the drive for bad sectors.
The second reason can be if you downloaded the Iso image of windows 7 from MS, and then burned it to a recordable dvd disk, you burned the Iso at a very fast speed and there are errors on the burn image. so re burn at a lower speed to avoid errors.
If u r hear any grrrr or scratch sound in hdd.are do u have one of these problems :hanging many times,many files corrupted,files disappear,or (A old bomb virus affected u r system can cause these like problems) unknown file automatically created with lenthy name and big file size above partition(ex:if partition is 60gb a file with 70gb size in 60gb partition).
if any of the problems u r facing in hdd no way to cure that if it has warenty contact manufacturer for replace.if no warenty throw it away dont use that because the virus affect inserted pendrives and all devices to malfunction.
no antivirus should find it and win xp sp2 is instalable in that hdd.
Otherwise dont care about these problem.