There is something worth mentioning if you had malware, system restore can still contain the malware so it is good practice to remove restore points to stop this happening. You can do this by turning off system restore and then turning it on again:
http://support.microsoft.com/kb/310405
Regarding your problem I am thinking along two lines here:
1) The download has been damaged producing the error message. As a test for this you can get a friend with iTunes working to download the installer for you. It is a simple thing to try and worth doing before trying the next step.
2) The download is OK, but windows has been damaged by malware and is failing to recognise that iTunes is OK.
This was why I got you to try method 3 in:
http://support.microsoft.com/kb/822798
However there are other issues possible issues and the page has a microsoft fixit that you could try.
However this strays into territory which is outside my knowledge and I have not tried the fixit myself.