Thanks for the info nobardin and sorry for the lack of info, I thought the minidump would provide enough information.
My system is self-build (7 years ago

)
CPU: AMD Athlon XP 2400
Video: Nvidia Geforce 6600
Network adapters: Linksys Wireless-G PCI Adapter
Realtek RTL8139 (disabled, currently running on wifi)
2 Hard disks: Excelstor Technology J240 (40 GB) => OS installed on this HD
Maxtor 6Y120L0: data disk
OS: Windows XP Professional with SP 2
RAM: 1024 MB Kingston
Virusscanner: AVG
Firewall: COMODO
Spyware scanners: Spybot search& destroy, lavasoft adaware
I had read some topics about these kind of crashes and i've updated all my drivers to the most recent available.
I've also scanned for spyware/malware (also with the tool you suggested), and scanned for virusses with AVG.
I ran the memtest but that revealed no mistakes in RAM.
Error from the event viewer is:
Error code 1000008e, parameter1 c0000005, parameter2 bf80487b, parameter3 bae6f46c, parameter4 00000000.
For more information, see Help and Support Center at fwlink
0000: 53 79 73 74 65 6d 20 45 System E
0008: 72 72 6f 72 20 20 45 72 rror Er
0010: 72 6f 72 20 63 6f 64 65 ror code
0018: 20 31 30 30 30 30 30 38 1000008
0020: 65 20 20 50 61 72 61 6d e Param
0028: 65 74 65 72 73 20 63 30 eters c0
0030: 30 30 30 30 30 35 2c 20 000005,
0038: 62 66 38 30 34 38 37 62 bf80487b
0040: 2c 20 62 61 65 36 66 34 , bae6f4
0048: 36 63 2c 20 30 30 30 30 6c, 0000
0050: 30 30 30 30 0000
The category is 102 with event id 1003, source system error.
Pinpointing the source is difficult because i have had in several occasions. I must say that 80% of the crashes happen when using a tool that connects to the network (surfing with firefox, starting msn, starting mIRC), but I also had it when playing a game which i installed without network options and can not connect to the network.
I have yet to try if starting in safe mode gives the error, but maybe you can derive some new information from this post

Many thanks for the help so far
