Try to lower your desktop and game resolution. Lowpoly models, change config to low model rendering. Use open-gl not direct-3d for HL1 games. Try out a default game config before change anything. New drivers installed? Maybe overclocking can help (if possible with celerons).
If you run your pc with at least 1gb there is no way defragmenting you hdd can bring you better or more fps in hl based games. Maybe faster loading for the engine and maps but when its done loading it depends on other parts like gfx, ram, cpu, ...
Also check your anti-virus anti-maleware solution and turn any system-memory watch-tools during playing off. It's just a single core low-end cleleron cpu, help him doing his job