Do not worry, I used to run old Roblox clients. This is just a side effect of them being meant for older Windows versions, a false positive, but note it may not apply to all clients since there are unofficial binaries---possibly with malware.
Make sure your PC supports the graphics. Can you supply a snippet of the line declaring the resolution, bits, and if it's Windowed as 3D or 2D? Maybe I can fix.
