Computers generally get slow on bootup and stuff if you keep them going without regular maintainence such as defragging and removing junk that is no longer needed.
It can also be down to how much space is on your HD. I generally try and keep 10GB free on my main HD, because if i go over the 10GB mark, it slowly goes to hell. Random crashes, 5 minute boot times, all sorts of joyous things.
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