The short answer is, you can't, becuase Windows (and other OS/devices) are trying to follow what was laid out by the IEEE society.
There is a quesiton about this over on ServerFault entitled "Can a WPA key be shorter than 8 characters?".
It seems technically you CAN, and it depends on how the WPA-PSK device's manufacturer interpreted the IEEE 802.11i-2004 document.
As the user who answered the linked question on SF points out, the document makes this 'assumption' when discussing security considerations in the "Suggested pass-phrase-to-PSK mapping" section:
A pass-phrase is a sequence of between 8 and 63 ASCII-encoded characters. The limit of 63 comes from the desire to distinguish between a pass-phrase and a PSK displayed as 64 hexadecimal characters.
So it seems you may be stuck until the WLAN is brought up to met that 'standard' of a min. 8 characters that Windows 7 (and apparently at least also Ubuntu) expects.