MBOX/Protools 6.7 works only once after install

[deedz remix]

Hi everybody,

After installing Mbox + Protools LE 6.7 on my Dell Precision 340 this application works fine for a whole session but after I closed it down and try to open it again it won't launch the program.
What happens if I double click on the protools icon is that it shows the hourglass for a little time and than nothing happens. Even after I completely re-installed my Windows XP Pro + SP1 and no other additional programs (i.e. third party plug ins etc) it still doesn't work.

I tried to install PT LE several times and I'm getting a bit desperate now. After searching the web and the FAQ I can't find anything about this problem. I'm I doing something really stupid? Something really obvious? Let me know!
Someone had the same problem?
Digidesign: sounds familiar? Please help!

Specs of my computer:
Dell Precision 340 Pentium 4 - 2.4 gHz, 512 MB of RAM.
Windows XP Professional - version 2002 - with SP1 installed.
PT LE installed on C drive - media is on other drives.

Thanks in advance - your help is really appreciated!
Merry Christmas,

Deedz