This is a common problem with paged pool memory and i had it too after i installed bf2 and the new patch a while ago it seemed to screw some thigs up for me somehow, a few solutions seemed to have worked for people on the net, i did a reinstall as my comp needed it anyway lol, anyway u could try any of these
update ur graphics drivers and make sure you have the latest version of direct x
Try the 5th post downs solution in this link >
http://www.neoseeker.com/forums/5571/t752500-failed-to-lock-vertex-buffer/2.htmAnd if all the others fail, re-install
If you want to back up your saved game data then backup the following folders for which game you want. you dont have to back up the cfg folders but its helpful as they it will keep all your settings (if you have changed any)
Half-Life 2
[STEAMAPPS]\[STEAMNAME]\half-life 2\hl2\cfg\
[STEAMAPPS]\[STEAMNAME]\half-life 2\hl2\SAVE\
Half-Life 2: Episode One
[STEAMAPPS]\[STEAMNAME]\half-life 2 episode one\episodic\cfg\
[STEAMAPPS]\[STEAMNAME]\half-life 2 episode one\episodic\SAVE\
Half-Life 2: Episode Two
[STEAMAPPS]\[STEAMNAME]\half-life 2 episode two\ep2\cfg\
[STEAMAPPS]\[STEAMNAME]\half-life 2 episode two\ep2\SAVE\
A full list of save locations for steam games can be found in the link
http://forums.steampowered.com/forums/showthread.php?t=840657Hope that helps