This is a normal occurance on computer operating systems that use disk for extended memory management. When you use your system, you have finite amount of physical memory (RAM). Let's say you have 128mb of RAM. As you open files and start various applications, the system needs more memory to perform those functions. The OS will determine what chunks of physical RAM are not being used and will "swap" it out to disk. Basically it takes the contents of that memory and writes it out to a disk file. It then will use the physical memory for whatever it needed it for. If the system finds that the application that was "swapped" is required again in memory, it will reverse the process by writing out what is in memory to disk, read from disk what was swapped before and then put it back in memory.
So, why is this going on after a period of inactivity? When the OS determines that chunks of swapped memory are no longer required, it starts to clean up the swap file by reading what is in there and making the swap file smaller. There is a confiluted algorhythm that the OS will use to determine what disk contents are eligable to be flushed.
Windows also does "stuff" (other clean up routines) when the system is unused and will take disk and CPU cylcles to do that. Also, check your scheduler - you may have the system update notification routine running (by default it is every 5 minutes). Some screen savers are also disk users - especially when they first start up.
"But I have found out from other expert sources that Rogers automatically detects when your PC is idle and they let other people use your modem until they detect activity."
There would be no reason for Rogers to do that. Your modem is for your use to the internet cloud. Other people can not use your modem for anything other than to gain access to your system.
If you leave your machine on all of the time with broadband though I would make sure that you have some firewall in place. You would be surprised what kind of things are going on out there.