100?C? Whoa.
You can do four things.
1) Clean the cooler. Use a soft pencil for that. Also, check to see if the cooler is spinning properly and check the cooler cables. Who knows if it's a bad plugged cooler?
2)Replace your thermal grase & above. You can buy those for like 10 bucks and they last much more than you'll even use. There are lots of videos in YouTube showing the how-to.
►
IMPORTANT, FOR REAL:
If the thermal grease is stored in a syringe-like tool, make sure you push the piston (I'm not a native speaker, sorry if it's not the name)
just a LITTLE BIT above a paper sheet or anywhere you won't do much of a mess. That's for avoiding you from having air inside the syringe, so you won't run the risk of having the silver nitratum "exploding" over your GPU with the air pressure when you apply it. Stop pushing it if the termal grease starts getting out of the syringe, then vuala!
►Make sure to see tutorials, if you apply too much, it wont work, if you apply too little, it won't work. You should apply a rice-seed-like amount of grease.
3)Replace your cooler & step 2. Of course you don't need to buy water cooling stuff (Unless you want). Just buy something that pumps a lot of air on that thing and it's solved.
4) Install one exausting cooler + One air pumping cooler in your desktop & steps above. This will not only cool the GPU, but also all the components from your PC.
IMPORTANT: Make sure you never trespass your power supply's wattage when installing new hardware. Check your current wattage levels here:
http://extreme.outervision.com/psucalculatorlite.jsp
I hope this helps.
