It is unclear what the OP's graphics card is.....Some R51s have ATI, while others have the Intel Extreme Graphics 2.
Google gives references for a Lenovo R51 as well as an IBM R51.
The Lenovo uses Intel but a link provided earlier gives drivers for AMD.
One way to determine what is fitted is to go Start - type dxdiag and press enter then look under the Display tab, but as the Win 7 Upgrade Assistant has given that the Video Controller (which is another name for the graphics card/chip) is not compatible for Win 7, then it's unlikely there will be Win 7 drivers for this machine.