It depends on what the specs are. How much memory and what processor does it have? I would say if it's really slow with Vista, 7 is not going to help much. XP will run pretty well on older Celerons with 512Mb of RAM. I think 7 needs 2Gb of memory to run well and at least 4Gb to run very well. I tested it and it's not faster than XP. I have a Core 2 Duo and 2Gb of memory and it runs OK though.
For 89 bucks, you can get a XP home OEM disk from the egg.
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MSI P55-GD65 with i5-750 @ 4.0Ghz vcore 1.370
Xigmatek Balder HS/2 120mm fans, Antec EW PSU EA750 750W
GSKILL Ripjaws 2x4Gb DDR3 1333
One 2Tb Seagate LP, two 1.5Tb LP Seagates
Gigabyte GTX 460 1Gb OC to within an inch of it's life
Lite-On 24X DVD burner, LG 12X Blu-Ray burner
COOLER MASTER Storm Scout
Win 7 Pro 64
Lots of fans spinning with little LED lights blinking
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