Guiminer problem

Hello all!

I started mining less than a week ago and everything was going fine until today.
Now everytime i click to start mining i get this error "NameError: global name 'GetLastError' is not defined" and guiminer doesn't start mining.
I've googled my problem but i can't find anything related to my situation, does anyone know how to fix this error?

I'm using guiminer in slush's pool.
OS is windows 7 64bits. GPU ATI HD5850.
Let me know if you need me to give you more info about my system.

This is the error that is shown on Console:
2013-05-11 16:27:25: Running command: poclbm.exe Aryetsok.worker1:pmac439277@api2.bitcoin.cz:8332 --device=0 --platform=0 --verbose -r1
2013-05-11 16:27:25: Listener for "Default" started
2013-05-11 16:27:25: Listener for "Default": 11/05/2013 16:27:25, ADL_Adapter_ID_Get failed, cutoff temperature disabled for 0:0:Cypress
2013-05-11 16:27:25: Listener for "Default": Traceback (most recent call last):
2013-05-11 16:27:25: Listener for "Default": File "poclbm.py", line 85, in <module>
2013-05-11 16:27:25: Listener for "Default": File "BFLMiner.pyo", line 48, in initialize
2013-05-11 16:27:25: Listener for "Default": File "ioutil.pyo", line 31, in find_com_ports
2013-05-11 16:27:25: Listener for "Default": File "serial\tools\list_ports_windows.pyo", line 182, in comports
2013-05-11 16:27:25: Listener for "Default": NameError: global name 'GetLastError' is not defined
2013-05-11 16:27:27: Listener for "Default" shutting down