Turn screensaver off! This is the biggest reason why SETI times are slow. I had a AMD T-bird 700@850 running with the screen save on and my WU's were 17 hours long. I turned screensaver off and now get 5 - 6 hours! Every time you go to install SETI@home it will setup the screensaver. It is very easy to forget this.
Get the i386-winnt-cmdline.exe! It has been proven that it will run 5-10% faster.
The best OS is Windows 9x! Try and run it on that operating system if you can.
Best processor is the Xeon with 1M or more of L2 cache. The second best is the Intel PIII. The third best processor is the Athlon. The reason Intel PIII is better comes down to FSB (front side bus). When you overclock the PIII you up the FSB. The WU is too large to fit in L2 cache sizes smaller than 1M. Therefore, higher FSB speeds help compensate for this by allowing the system to access the main memory much faster.
Overclock your PC to the max! A faster processor will complete a WU faster.
Good, quality RAM is essential! An example of good, quality RAM is Micron CAS2. CAS is a latency measurement and CAS3 would be slower.
Disabled Shadow system Bios, Shadow video Bios, and Cache video RAM. Those setting will help speed up your SETI times.
Run SETI on more than one Computer. With SETI, you can install it on as many computers as you want. Use the same E-mail to set up each account and all WU's will be added to your statistics.
Turn off all programs that are running in the back ground when not using your computer. Click Control-Alt-Delete to see what is running and turn off the programs that are not needed to run.
Turn off power management. You can do this in the control panel by clicking on power management and setting everything to none