A couple of thoughts come to mind.
Since the system supports a max of 2GB of RAM, it's probably a single core CPU. Vista and beyond were written with dual, quad, or more core systems in mind.
Following on the same basic theme, odds are the integrated video on the laptop is rather old and not up to the task of handling Vista's Aero interface.
Another very distinct possibility is that of malware, possibly even combining with one or both of the above to compound things even more.
That should be enough to get you started looking. And if this laptop came with Vista, check for a "Vista Capable" sticker. If you see one, then you're screwed. That just means the system is, as it says, capable of running Vista. Note it says nothing about being able to do it well. That was generally the idea behind the "Vista Ready" stickered systems. Intel didn't want to stop selling it's crap low end integrated graphics so soon, so they pressured Microsoft to come up with the Vista Capable program, which I believe even prompted a few lawsuits. I also personally think it has a lot to do with the bad rap Vista got with people. They bought these low end POS systems that were "Vista Capable" and blamed the OS instead of themselves for buying a cheap, low end, POS. In any case, if you see that sticker, you may as well just give up trying to improve performance to any significant degree. You can turn off the Aero UI and drop down to something more like Windows 2000, which is certainly more within the realm of reason for a lot of lower end Intel graphics chips, but it will only do so much.
A friend has an HP laptop running Vista Home Premium with 2GB of RAM (max supported by their system). In many cases, when they click on something, it takes several seconds before they get any response. Once they are in an app, it seems to work fine. It seems most noticeable when clicking the network icon to find available wireless connections. It takes a while before the pop-up displays that allows you to even say you want to see the list of available networks.
Any thoughts on why this lagging is occurring?