1- Is your computer running properly overall ? Can you play videogames ?
If not, the issue is not with Guiminer but it is possible that you burnt your videocard.
2- Did you monitor your temperatures while mining ?
Mining runs your GPU at 100% all the time. It is easy to burn out a card.
3- Nvidia cards are fantastic for games, but are lousy at mining.
I am sorry, I don't think those are the answers you wanted to hear.
If you are still interested in mining, I strongly suggest finding a used computer with a low end cpu... such as a Pentium4 or old single core AMD Athlon with 1 or 2 PCIE slots on motherboard... you can find these for 100$
Make sure the PSU is at least 450 to 500W depending on the GPU you will put in.
Use an ATI/AMD videocard.
https://en.bitcoin.it/wiki/Mining_hardware_comparisonFind a good used one that has a fair hash rate.
Good luck