@Hardware not capable
First of all, a "cheap" Acer notebook will never perform properly. It will have a low-end chip, little RAM, and probably a slow disk that's not made by a good manufacturer (likely a Hitachi).
Second, a Sempron is not a fast processor and 1GB of RAM is insufficient to properly run Vista. Realistically, that machine should not have been shipped with Vista.
To run Vista (on an AMD notebook), it should have had a Turion processor and 2GB of RAM, and preferably not been made by Acer.
As to your claim that upgrades for laptops are not cheap, actually, when it comes to RAM, they aren't any more expensive these days than for desktops (at least, not in North America at somewhere other than your local big-box store [e.g. Best Buy, Future Shop, etc...]).
In addition, there is one performance eating element of Vista which you can't turn off... Indexing. This combined with memory paging due to the amount of RAM is probably why that machine is failing so hard.
Honestly, this is my biggest grievance is that people can't be bothered to figure out what they need to properly run something, then gripe about it once they buy something underpowered and find that it's nto running it in the manner they expected.