Originally Posted by nitrofreak
Try holding the Ctrl key and pressing escape. If this doesn't work, open task manager and go to the File menu, select New Task (Run...) and type explorer.exe in the resulting dialog box and press ok. If neither of these work, you will have to put the drive in another computer or use a tool like bartpe to recover the data. This is almost definately the result of a malware infection, though
Nitrofreak is on the right trail here.
If you open the task manager (ctrl-alt-del) you can click File > New Task, type in Explorer.exe and the desktop should launch.
However if it fails, then most likely it is a failure caused by a 3rd party shell extension.
If this is the case, go to this link: http://www.snapfiles.com/get/shellexview.html
Download shellexview to your your computer and extract it from the zip file. Copy it to a floppy or burn to CD and transfer it to your sisters system.
From your sisters computer open task manager and click File > New Task, type in CMD and click OK. This opens a command prompt. Change to the drive letter where the CDrom or Floppy is located and run shexview.exe
with this tool, you can select and disable/enable any shell extention. You should ignore anything called a "windows shell common dll" because those are rarely the cause of the problem. Instead look at antivirus related shell extensions or anything that seems out of the ordinary. Once you've disabled everything that looks suspicious, try launching EXPLORE.EXE again and see what happens.
If this fails, your only recourse is to get a someone to look at a usermode memory dump. Unfortunately I'm really tapped out on time of I would offer to look at it....