Help me solve my problem please!

Having problems with my computer it often crashes for unknown reasons, e.g. im running the symnatec virus scan and limewire(downloading program) and it crashes, sometimes im just listening to music and browsing websites, sometimes it just crashes. any ideas why? Here are my specs if you need further informaton to help you to help me just ask.

Operating System : Microsoft Windows XP Home Edition (5.1, Build 2600)
System Manufacturer: SiS
System Model: 730S
BIOS: Version 1.00
Processor: AMD Duron(tm) , MMX, 3DNow, 1.3GHz
Memory: 512MB RAM
Page file: 128MB used, 1121MB available
DirectX Version: DirectX 9.0c

Graphics Card is

Name: ASUS RADEON A9200SE
Manufacturer: ATI Technologies Inc.
Chip Type: Radeon 9200 SE AGP (0x5964)
DAC Type: Internal DAC(400MHz)
Approx. Total Memory: 128.0MB
Current Display Mode: 1024 x 768 ( 32 bit ) (85Hz)
Monitor: Plug and PLay Monitor

Main Driver: ati1dvag.dll
Version 6.14.0010.6404
Mini VDD: ati2mtag.sys
DDI Version: 9

Sound

Name: Sis 7018 Wave
Driver: sis7018.sys
Version: 5.10.0000.6180