All you can really do is lighten the load (Shutdown all unnecessary background processes)
For example go into Start/msconfig and uncheck on the startup-tab everything that isn't Ant-Malware or otherwise absolutely necessary.
If you can't add more RAM perhaps starting demanding games with a batch while shutting donw wxplorer for the time being
taskkill /F explorer.exe
(might need some fine tuning...it's just a concept.. I haven't tried it out yet.
Save it as hl2.cmd, move it in your Half Life directory and start if from there. That should give you some 30, 40 MB more not much, I know