When was the last time you or your girlfriend cleaned up that laptop of dust?
If the answer is 'never', then get to it.
dust has a tendency to clog laptops over periods of time, and various problems can occur due to this.
That's one possibility.
Second one is linked to potential licencing issues, or just a plain bad install.
Vista had a tendency NOT wanting to install various updates.
What you could try doing, is downloading a Vista with slipstreamed SP2 (32bit or 64bit - depending on which one you had) and an edition that originally came with the laptop (Home Premium, Ultimate, whichever -though in case of computers, I think they usually ship with the Home version).
Anyway, you can use the OEM key on the back of the laptop (which is interchangeable between 32bit or 64bit versions, but not between editions - Home Premium, Ultimate, etc) to register your installation and that's it.
By installing Vista with slipstreamed SP2 (you will get majority of updates onto your computers), you avoid the hassle of downloading the service packs and individual updates on your own, and merely will have to install some updates post SP2 (but that's it).
Keep in mind that Vista by default CAN act slower than say Windows 7... although it may also not (because, in most of my experiences, a clean install of Vista isn't that much slower from clean Win 7 install).
It varies from system to system, but I would surmise that some of the slowness stemmed from the DELL preinstalled bloatware programs.
You would be surprised just how many programs will be starting up with Windows on a pre-installed system....
And even if a program doesn't have a startup entry, it may still create additional services which will inherently slow the system down.
A clean install of Vista with SP2 slipstreamed onto it would probably work very close to Windows 7 in terms of speed, though the latter will likely end up more responsive (and is inherently more optimized).
Finally, I wouldn't completely discount the option that your gf's hardware might be dyeing.
The HDD could be the one that is on it's way out... but even if it is, replacing it shouldn't be difficult as you ony need to tell us which model is that Inspiron and we can track down compatible (and probably faster) HDD's for you.
:-)