my iMac, 2 GHz Core 2 Duo, 1 GB 667 MHz DDR2 SDRAM, has been running slow ever since I installed Snow Leopard. Opening applications takes much longer than ever. I have repaired permissions several times but I always get the following message.
Warning: SUID file "System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDAgent" has been modified and will not be repaired.
Any help is appreciated, thanks in advance.

You can safely ignore that message. It is normal. Even if it weren't, a bad permission won't usually make your system slow. It generally makes things not work.

As to why your system is slow - have you looked at Activity Monitor? It's in the Utilities folder. Take a look at that, make sure the drop-down is set to "all processes", then sort by %CPU (click on the heading) and see what is taking up all your CPU.

The problem may lie in the fact that you only have 1 gig of RAM in your Mac. Snow Leopard wants 1 gig of RAM just for the OS. Many people are finding that some Apps are running slower than before. This is the reason...

My suggestion is upgrade to the largest amount of RAM your model will take. I have found that macsales.com has quality RAM for the best price currently.

I'm with Dave on this one. 1 gig of RAM is not enough, and last I checked, prices were still way down.

thanks everyone. the activity monitor looked ok so I will order some more RAM

Well, it wouldn't hurt to look at the memory portion of Activity Monitor and verify you need more RAM (but of course, you *always* need more RAM anyway...).