Least likely is Malware.
Check the S.M.A.R.T. status of the drive in Disk Utilty by highlighting the Drive & looking at the bottom of the window.
Open Activity Monitor in Applications>Utilities, select All Processes & sort on CPU%, any indications there?
How much RAM & free space do you have also, click on the Memory & Disk Usage Tabs.
In the Memory tab, are there a lot of Pageouts?
sorry I am long in getting back to your reply. Office computer and I was not in the office this weekend.
This is a Mac Mini 2 Ghz Intel core Duo, 2GB 667Mhz DDR2 SDRAM, 4MB L2 cache, external 250GB USB drive
At test time running Acrobat, Firefox, iPhoto, Utilites
Results of memory, CPU and disc stats
System Memory - Free 168.8MB, Wired 260MB, Active 1.16GB, Inactive 430.8MB, Used 1.84GB, VM size 176.55 GB, Page ins 4.75GB, Page outs 2.86GB, Swap used 1.65GB
Disk usage Utilized 52.42GB Free 67.27GB
CPU% use Top users - Activity monitor root 6.7%, Firefox 0.6%, Acrobat 0.4 over course these %'s change as you look at them
Right off the bat, looking at your page outs tells me you don't have enough memory for the tasks you run. OS X writes the contents of memory to the hard drive when it needs to free up space to run something else. Too much of this can impact performance. Guideline is that page outs should be no more than 10% to 15% of page ins. Yours is 60%.