Linux!!!!
A new version of Ubuntu with docky installed would probably feel more at home for her after using osx, would also solve the slow and unresponsive issues. Depending on the components of the dell, a hackintosh may also be possible.
I use Elementary OS Linux on my older laptop... it looks very much like OS X. People often think I have a hackintosh as a result.
As for OP, I've used the lot of Windows, OS X, and a few flavours of Linux, fairly extensively. Long-term, I like Windows 7 quite a bit. I'm not fond of 8 yet, but Microsoft seems to have a trend of every second iteration being solid.
The only thing is that I find I have to keep it pretty clean. I don't bog it down with all kinds of stuff running in the background, and I tend to reinstall fresh (or reimage the work machine) when it starts to get gummed up. I only use the Microsoft Defender / Security Essentials.
I'm thinking that if she finds her computer slow, she might have a 5400 RPM hard drive with a heavy encryption. My work laptops are like that and despite being powerful machines, the hard drive (and the encryption/antivirus/spyware) reduces them to a crawl. If this were my only experience with Windows 7, I'd probably dislike it too.
I've also found OS X is not without its faults. The silly "maximize" behaviour aside, I've had to completely fresh install the OS on our late 2013 Macbook Air when it was only a couple of months old. Once was the keychain got totally buggered up from changing an account password, and it was impossible to repair.
