search forum for tethering bold, put in a string manually not copy and paste under standard modem in device manager or whatever its called. Then create a new dial up connection, choose standard modem and enter whatever the threads says your OP's settings are, crackberry has most of the OP's dial in numbers and passwords listed too I know. Then disabled IP header compression and 3 others in advanced settings of the dial up connection. Make sure you have blackberry device thing opened when dialing in, not desktop manager, the device thingy.
I have written a forum post on the settings to tether your Blackberry as a modem to Windows XP. its on the this site Tech Steps | just click on forums. I also have a blog post on common uses for this method. enjoy! hope it helps