1st of all, you have to somehow verify that temp reading. 192F (89C) teeters on a complete meltdown. A crude way to check is to get your system up to temp, then shutdown, unplug it, quickly remove the side panel & carefully touch the heatsink...it should be warm, but not hot. If you can't keep your fingers on it, you have a problem.
Make sure the heatsink is installed correctly...there's a step in the bottom that is supposed to go over the end of the CPU socket...if you install it backwards, the heatsink won't make full contact with the CPU core.
Radio Shack paste is fine, but just like any other paste, it has to be applied properly. You should have used a dab about the size of a BB & spread it thinly & evenly over the core. Here are some excellent instructions:
http://www.arcticsilver.com/arctic_silver_instructions_small.htm
You should also have case fans...one intake in the front & one exhaust in the back. In the meantime, try running with the side panel removed. The system temp should be as close to room temp as possible.
ASUS A7N8X-X
Athlon XP 1800+
8.5 x 200MHz
1024MB PC3200 2.5-3-3-7
Asus A9550GE/TD 128MB
WinME/WinXP Pro SP2