I have an older computer and it seams to run windows 7 fine without the graphic enhancments. I was wondering whether I can run Windows 8.1 RT on it.
did you say RT? is your processsor's architecture ARM?
Running the Upgrade Assistant as explained in Oron's post will be your best adviser given the fact that you have not told how old the machine is.
well windows 7 by default uses Aero Glass Transparency whereas windows 8.1 dont, so there is a chance that you can install windows 8.1. Windows 8.1 is faster than windows 7. As always do a system image before trying to install windows 8.1.
I think you are slightly confused about the versions. Windows 8.1 RT is designed to run on tablets only, not on PCs (see the FAQ).
That said, the full version of Windows 8.1 will probably run just fine on your PC, as the hardware requirements are actually fairly low (1 GHz CPU , 1GB RAM, 16 GB space on HD and a Microsoft DirectX 9 graphics device with WDDM driver). Just to be on the safe side, download and run the Upgrade Assistant and it will tell you if your computer is compatible.