Nokia N900 - Windows firmware upgrade via flash tool,
I downloaded the firmware, installed the flash tool, put the firmware in the same directory as the tool and then run the folowing command

C:\Program Files\maemo\flasher-3.5>flasher-3.5.exe -F RX-51_2009SE_3.2010.02-8_P
R_COMBINED_MR0_ARM -f -R

and I receive this message

flasher v2.5.2 (Sep 24 2009)
RX-51_2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM: %m
C:\Program Files\maemo\flasher-3.5>

The phone is connected via USB and I have pressed the U button before pluging it in untill I get the nokia logo on screen

Type the full path to the BIN file or copy it into C:\Program Files\maemo\flasher-3.5\
Edit: Also make sure to use the correct filename, to ensure you type it correctly use tab
flasher-3.5 -F RX(PRESS TAB) -f -R

You could also try renaming it to use a shorter filename - there's been some reports of systems having issues with the full filename (I think Windows has a maximum path + filename length).

Thank you all for your help, moved the files/folders to

C:\maemo\flasher-3.5 as I noticed that windows didnt like the space in the path "program files" and renamed the file to "FW.bin" and with the use of the full path the problem was resolved.

Thank you all once again, I just got the phone today and wanted to upgrade it before using it as I always do with my nokia phones, now I can start configurating it as I like it

Use speech marks to enclose a space in a filename and windows will be happy.