Why do my Instrument paths disappear in cubase sx-3?

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I could be recording with no problems for days and then the next day when I load my song,any song, in which everything was fine, I get a list of plugins not found. I go to the plug-in information window where all my instrument locations were set and saved with the song (so I thought)and sure enough,C\Programfiles\plugins is the only path left.
Very frustrating to have to redo all my instrument locations. I've even tried system restore. nothing.
I'm using sx-3 3.1, dual amd opteron processors, windows xp, 3 gigs ram.
Would appreciate some input.
thanks
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[dblock]

That's dual 2.0 opteron processors. sorry