I don't have a SlingLink device myself but I'm guessing it's exactly like the D-link powerline adapters I have.

Yes, that's more-or-less correct, someone would have to plug into one of your power outlets. I've done that myself on haloween so I could get networking outside - this was before I had wifi. I plugged into a outlet on my front porch so I could surf the net inbetween handing out candy.

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As for running the utility on Vista, I suspect it only needs a network connection so you could use a virtual machine. I do this to run XP-only software on my Vista computer, and for configuring my routers and print-servers that aren't fully compatible with Vista and Internet Explorer 7.

Download Microsoft Virtual PC 2007 (free for Windows users)
http://www.microsoft.com/downloadS/details.aspx?FamilyID=04d26402-3199-48a3-a...

Download the IE6 compatibility test image (free)
This is basically a copy of XP Pro that expires every 3-6 months, at which point you have to download the new version.
http://www.microsoft.com/Downloads/details.aspx?FamilyID=21eabb90-958f-4b64-b...
You only need the first file, it's called something like IE6-XPSP3.exe

Install/extract those and run Virtual PC. Create a new "Virtual Computer" per the setup wizard.

Last, configure the Virtual PC machine so it has a direct connection to your network:
1. shut down the virtual machine
2. go to Settings
3. On the "Networking" tab, select the network adapter that you're connected to - that way your virtual XP computer will be able to see the SlingLink.

Then, you should be able to install/use the SlingLink software within Windows XP as if you were using a real computer with XP. Just think of it as a whole seperate computer within your normal computer.

[edit]
Just wanted to add, I was able to install and run the SlingLink software on my XP Service Pack 3 virtual machine without any trouble. Although I can't test it since I don't have a SlingLink, I believe it would work perfectly as designed, using the above procedures to run a virtual machine.
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Hope this helps!