Lock On is a very demanding game for your computer, people with better systems than yours have been complaining for over a year. Your system is above the bare minimum but below the recommended specifications. You can disregard published minimums, double them for good performance. You're going to have to turn down the detail and the features to the minimum to get it to work. Even then, it may perform so poorly that you won't enjoy it. For games of this nature, you need a better system. I would suggest a minimum of 1 GB of RAM, min. processor of 2.5 GHz and a 256MB video graphics card. Falcon 4.0 is similar to lock On and runs well on more modest systems.
Lock On
Windows(r)98/ME/XP/2000
Pentium III 800/AMD Athlon 600 or
better
256 MB RAM
(512 MB RAM recommended)
3D video card (DirectX 8.1 compatible) w/32MB RAM (128MB recommended)
Sound Card (Direct 8.1 compatible)
DirectX 8.1 or higher (included on disc)
4X CD-ROM or better (Not recommended for use with CD-RWs)
Internet connection (56 kbps or better) or LAN for multiplayer
Hard Drive Space 1.1 GB