Failing ram is the likely cause. These are big files which use memory buffering heavily to copy to your disk, if the ram is faulty or corrupting the data somehow, you will get a duff copy on your hard drive. The install program checks the CRC of the file it has copied to your hard drive against the one on the DVD and if they don't match, it throws a tantrum.
Had similar issue with a mix of ram with different timings. We tested with memtest86+ for a couple of days with no problems reported but when we took out one of the ram modules, everything worked fine.
If you have access to a file compair utility; try manually copying a large file (600mb+) from any other disk to your hard drive and compare the two files. If they don't agree then chances are you need to play around with your ram or get new modules.
Other possibilities include; a virus, any number of other hardware issues or phases of the moon.