I am running the red hat linux in GUI mode,

I am doing nothing with the computer, but the HD light is flashing continuously.

Why? How can I know what is the computer doing at that time ?

This sounds like Cron is running its scheduled jobs on your machine. Cron will run until the scheduled jobs are complete. If you reboot or switch off your machine whilst Cron is running, Cron will run again when the machine is up and not being used.

You can scheduled these tasks for another time if you want. Cron is the daemon to execute Scheduled commands. Dont worry about this really, unless it is adversely effecting peformance. The particular Cron jobs that are running are the preset ones from Redhat. You can use Cron if you want to run scheduled backups etc..

At the console type "man cron" for further info.

I hope this helps.

It may be indexing the files too. It may also be updating the journal and cache. Mine does that pretty regular.

I seriously doubt you have a virus in Linux, unless you put one on.

Hope that helps