Rob,
You said it has a v201 card. That means in order to run USB it has to be converted from serial to USB via a converter. Get yourself a new
IOGear USB to Serial adapter they are cheap @ around $15 Prime. These are great converters, but their weak point seems to be the ground wire inside the size of a cat whisker, which invariably snaps or causes intermittent connection reliability.
As to the power supply, yes, you can certainly upsize it with no compatibility issues. As long as it is ATX spec and has the connectors you need for the MoBo and peripherals, it's good to go. This would most likely fix your old XP machine (unless the HD is fried) - but it doesn't explain why you were not able to connect via Win7 laptop. This is why I am pointing at the USB converter. They can go bad without warning and cause the issues you describe.
In terms of running Win7 and connecting, my personal preference is to run an archived version of SB3. I personally like v3.5.20 on my PRTAlpha7.2 machine, which you can download
here. As long as you are not running Link, there is little to no benefit in running a newer version, in my experience. In fact, the newer versions don't move the tool in the way that I prefer because things like keyboard operation put the focus on Desktop tools and how they work over the large gantry tools. (Packet size is larger for higher UV DT tools which makes the Z axis on gantry tools move a minimum of 1/2" at the mere tap of the PgDn key - certainly annoying!)
Hopefully you get things resolved. At the very least, if the USB converter proves to be OK, you have a spare for the day when it just dies without warning...