If you originally install your app as admin, then you'll have to make the update as admin.
If you install it as a regular user, then set write permissions for the current user for your files. Then it is possible for the same user to update the app.

On Vista you can elevate you app's rights to a more privileged user ( by asking the current user a user name and pwd ). You'll just have to add a few extra entries in the app manifest and some lines of code.

Regards