I have a AMD processor 700 Mhz with 1Gb ram, my system problem is that when i do a multitasking work my system strucks and i have to restart my system

Well firstly, your RAM is more than sufficient for multitasking so that's not the problem. You could however do with a slightly faster processor. There is a full list at Wikipedia, this link will take you straight there. I would personally recommend at least a Duron 1100 at 1.1GHz. Because this range of processor is a single core, it will struggle when it comes to multi-tasking. I will however congradulate you on using AMD, I find these to be the best value for money and are great performers.