Okay, I don't know if anyone has encountered this problem before. Furthermore, I am not sure whether I am able to express the problem I have very well here. But I will try my best.
Basically, this has to do with the variable $DOCUMENT_ROOT with regard to PHP. I am using this tag on my main blog page as I have to call an external script (non WP-related) from another folder in my base directory.
When I was using 126.96.36.199 without the new coding in wp-settings.php where it disables the "register_globals" setting, my script was able to work fine. Mainly because the $DOCUMENT_ROOT was directed to http://www.mydomain.com.
After upgrading to 1.5.2, the script I am using suddenly stopped working because the $DOCUMENT_ROOT was now directed to http://www.mydomain.com/wp-content/themes/carrots/
Assuming that the above problem was attributed to the new lines of code added in wp-settings.php regarding the "register_globals" security fix.
I managed to solve the problem temporarily by replacing my wp-settings.php with the old file (From 188.8.131.52) while the rest of my WordPress is running on the 1.5.2 files. However, I fear that this problem may recur again when the next release comes out.