Try 1024 x 768, supported video resolutions can depend on many things. You might also try updating the video drivers. You can do this by clicking Start -> Control Panel -> Settings -> Display -> Click Settings Tab -> Click Advanced and look around in there for an update drivers button.
Off topic, you should should upgrade from glquake to GL ProQuake. ProQuake is a modestly bug fixed authentic improvement to GL Quake.
ProQuake web site.
http://www.proquake.com
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If you are into fancier graphics, you might try Qrack or JoeQuake. Regardless of what you use, GLQuake has serious issues.
JoeQuake info page:
http://forums.runecentral.com/viewtopic.php?t=245