Premiere Pro and After Effects are 64 bit only.
If you're running Windows 7 32 bit or Windows XP 32 bit, you will not be able to download or install Premiere Pro or After Effects.
"Why is Premiere not listed in Application Manager?"
If you're running Windows XP, you also will not be able to download or install Lightroom 4
Unfortunately, your only option is to buy a new 64 bit computer.
As John said, if it doesn't show up, you system doesn't meet the requirements.