Windows BSOD Problem.Pls HELP!!

Hi!
Im using Vista Ultimate SP1 right now.Im using a USB modem to connect to Internet.Windows BSOD often appears & my PC gets restart.When shutting down it shows message Dumping Physical Memory To Disk.After restart it shows message "Windows has recovered from an unexpected shutdown".On clicking detail,it shows the following:
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Problem signature:
Problem Event Name: BlueScreen
OS Version: 6.0.6001.2.1.0.256.1
Locale ID: 1033

Additional information about the problem:
BCCode: c5
BCP1: 00000000
BCP2: 00000002
BCP3: 00000001
BCP4: 81B36218
OS Version: 6_0_6001
Service Pack: 1_0
Product: 256_1

Files that help describe the problem:
C:\Windows\Minidump\Mini091309-01.dmp
C:\Users\R@z0r\AppData\Local\Temp\WER-33087-0.sysdata.xml
C:\Users\R@z0r\AppData\Local\Temp\WERC80F.tmp.vers ion.txt
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I have the same problem with Windows XP.This problem is since i am using USB MODEM & installed USB MODEM drivers.If anybody know how to fix this, please refer to me.I'll be grateful to him/her.
Thanks in advance.