The default gateway is not available

Hello, after checking older threads about this problem, I still could not find a way to fix it.
About a month ago my computer started giving me this message, and was disconnecting every few hours, usually when I download or stream. Lately the problem is much worse, the internet disconnects every few minutes, even if I am not downloading anything! (I cant even write this post for the computer, because everything works so slowly and disconnects so often).
I am connected through Siemens SL2 141 router (wired connection), my network adapter is Marvell Yukon 88E8053 PCI gigabyte. I have 3 other computers that are connected to this network, 2 wired 1 wireless, all of them are working fine. (running XP and not windows7, though for 2 years it worked fine).
I also included my spec.

Hope you can help,


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