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jilong,
I want to preface what I'm about to say with the statement that I am not a DBA or even remotely a skilled DB coder. I've tinkered with both MySQL and MS-SQL, but the basics is about as much as I know.
That being said... I can tell you that any kind of "on the fly" DB conversion is a bad idea. This adds a layer of complexity and server load that will slow your site down and offers little to no benefit.
From what I understand of the two DB engines, the primary difference between the two is syntax. With that in mind, you would think that it would be simple enough to convert XOOPS to MS-SQL, but it is not. It's not just the core XOOPS application you have to be concerned with, but also modules. Most "full-featured" modules have their own DB functions and calls. Therefore, you're in for a lot of code hacking if you want to use MS-SQL.
A better solution, IMHO, if you do not have MySQL support, is to seek a free or paid MySQL DB server host. There are providers out there who specialize in just providing DB hosting (not including web files like a traditional host). You could try searching for "MySQL database hosting", "free MySQL hosting", "free database hosting", etc... in Google and you will come up with several results. This will allow you to use XOOPS with its native MySQL support without adding additional load to your server and page load times. One such service is
FreeMySQL.net. This is by no means an ideal situation, but it will get you by until you can arrange a better setup.
Hope this helps!
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.