It could be and in answer to your next question Infections can survive a Format if they are designed to and because a Full Format only touches every third Sector of the HDD, a Fast Format only addresses the Master Boot Records and doesn't overwrite any other part of the HDD.
If you have an infection designed to survive a Format you need to wipe the HDD with a Wiping Utility like Boot & Nuke
http://www.dban.org/download
of Kill Disc
http://www.killdisk.com/downloadfree.htm
To test your HDD you should use the Makers Testing Utility not any Supplied by another maker. These are available from here
http://www.tacktech.com/display.cfm?ttid=287
These typically run from a Boot Disc not from within Windows.
As for the rest I'm not exactly sure what you mean here the Memory Usage is always going to increase when you open any application and shrink when it is closed. The more load you put on a system the more RAM it is going to use and depending on the actual system specifications this may change by just running the system. If it is low on RAM the HDD Space Used is going to increase when you load more things as once the RAM is fully utilized items will be written tot he Swap File that Windows Maintains on the HDD sometimes called a Page File depending on the OS in use.
I hope that goes some way to explaining how a Windows System works.
Col
