Its obviously some conflict with your PC hardware/software because i never have a problem updating/installing new OS including OS5 beta or otherwise... i always say on these occasions, do a clean re-install windows as a last resort as this will guarantee to clear any remnant files on your computer. I know its a harsh answer to what should be a n easy solution, but you can spend several hours/days trying to solve these issues, and its simply best to invest your time getting your PC to a clean state before installing virus software.
I run win7 (64bit) and never ever had issues like some people on here, so its not access's fault it wont install on a corrupted PC is it??.. just annoys me the first person to get the stick is access!.. get your own house in order first, as MOST problems with virus are down to users PCs/Macs and not access software.
Hope that helps...
My day job has been working in IT for the last 15 years. If you look at my post on page 2, the machine I am installing TI OS5 on was JUST rebuilt with a fresh install of Win7 Ultimate 64-bit. There is nothing on this machine but pro audio hardware and software. The installer for 4.5.3 works just fine. Believe me, there is something wrong with this installer. Maybe not for everyone, but in some situations.