Hi,
Try to study the microsoft articles carefully, you will find them in:
http://support.microsoft.com/kb/325023/en-us
Although I never did an upgrade from MSDE to SQL 2000 Standard or Enterprise,
the microsoft site suggest it can be done.
Be carefull before you do this and be sure you alway can go back to the
original situation of your system and data.
If the data is to valuable, I would suggest to migrate towards a new installed
server (new hardware/or virtual machine) and make a migration plan.
Cheers
Arthur