you don't say what version of the iMac, how much memory you have installed or how much free space you have on the HD so it is difficult to determine whether the slowness of your machine is due to a slow processor, lack of memory or another factor.
So let's go with what we know. Deleting files, history and favorites folder is not going to make your machine run any faster, unless your problem is related to lack of memory and HD space. Under OS X, it is necessary to defrag the drive, which is just as well in your case.
Regarding the USB to SCSI adapter. I have seen a couple of devices, but have had no experience with them. One is made by Adaptec and the other by Cables to Go. Neither of them mention support for OS X It would probably be cheaper/easier to get a USB or a Firewire Device rather than fight your way through the SCSI jungle which can be difficult.
With a little more information, we could probably move on with the slow problem
Hope this helps so far
I just want some advice on my Imac
It is running really slow; despite me having deleted a lot of old files and runing a virus cheack regularly.
It is running on OSX 10.2.5 and when software update tries to download the next version up, it says it will take 30 hours to download!!
I am only on modem, so I cannot stay connected continuaslly for this long.
I don't know how to defrag, or if you even have to do that for Imacs. I have cleaned out my history and favorites folders and some of the caches, but it makes no difference.
I do not have a cd burner, so every thing is stored on the computer itself.
What else can I do????
Please help, getting very frustrated!
Also, is there a cord that connects scuzzie apliences to USB computers? I have CD burner and zip, extra hard drives etc, but they are all the old pin and hook connections!