Disappearing icons, taskbar and program.

I'm using a shared computer with 4 different accounts: two personalized accounts that are locked by passwords, a single general account and lastly, a guest account.

The problem I've encountered started yesterday, when the user of the first personalized account told me that her account was missing icons in the start button window. She fixed this by deleting and re-creating her account, since restarting didn't work, and she had backups of her files in a separate drive.

However, I suspect that the same problem persisted, and only a few hours ago I discovered that the Windows Live Messenger program on the general account had mysteriously vanished, leaving only the ordinary Windows Messenger. This too was easily fixed by downloading the installer, and initiating a repair function that came with it. The most recent event involved a missing taskbar in the 2nd personalized account, which was fixed by a restart.

I use Firefox for browsing the web, and so far only one known piece of Adware has entered the system, which is the MyWebSearch search bar.

My question is, are these problems pointing to some kind of malicious program or virus? Any help will be appreciated.