I would not touch a used power supply with a ten foot pole. All the analog components in the power supply age.
Denholm, switching power supplies are supplies that use electronics to shape the power output wave. As opposed to linear supplies which simply include a huge transformer and rectifier to shape the output wave. Switching supplies provide a more stable output that does not vary much. Linear supplies put out what they get in. If the input varies, the output will vary the same amount.