is it a CRT ?? (i guess it it one) so, maybe its nearing its life, i mean the picture tube's a.k.a CRT = cathode ray tube's life..
Hv u tried setting it to a different resolution (to make sure its not a reso problem) ? also, do u see this problem of lines even at boot time or bios or jus wen the windows has been fully functional ??
if the problem is constant throughout, im sorry u dont hv any option but to look for a change..
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CHeers n e-peace...