My main workhorse computer is an AMD Sempron running at 1.33GHz (supposedly) and although I no longer render with it I am still able to work with very complex files without the sort of issues you are facing. For example, one project I am currently working with (Transneptune Triton base exterior) is currently over 137MB with no major speed problems. The only time I ever experienced issues like yours was with a castle project a few years back that topped out at over 287MB before I abandoned it, and in this case the issue turned out to be memory (or lack of it). Upgrading from the original 500MB to 1.5GB RAM fixed the problem.
When your system runs out of RAM it will make use of 'virtual memory', which is essentially an area on your nominated hard disk(s) that can be used as temporary storage. The access speed of your hard disk is WAY slower than RAM and will have a catastrophic effect on performance, which sounds like what you are experiencing. As Raxx mentioned, upgrading to 2GB should be fairly cheap these days, though I don't know much about Dell laptops and how upgradable they are.
To check if memory is the problem, open Anim8or and load the file that is causing the problems. Then open Task Manager (Ctrl+Alt+Del) and open the Performance tab. In the Physical Memory section check your Total (should be around 1000000 on your system) and the Available figure (circled in red in the attached screenshot). If your available memory is low (approaching zero) your system will be relying on virtual memory.
Aside from adding more memory (I would recommend 1.5GB as a minimum for Anim8or) you can make sure nothing else is running that doesn't need to be. Also in Task Manager, in the Processes tab, sort by the Mem Usage column (click the title) and you will be able to see the processes that are using the most memory. You might be able to close some of these (highlight and click on End Process) but be aware that some are important for the correct operation of Windows. If while closing processes things start acting weird just reboot and all will be well. The biggest culprits are likely to be antivirus software and other such 'memory resident' entities, which could be closed so long as you disconnect from the internet temporarily (don't be connected to the internet without AV software, reboot before reconnecting).
Along with the previous suggestions re optimising and reinstalling a fresh Windows copy, in my experience the biggest factor in slowing systems down is insufficient memory and if you are able to manage it I would definitely recommend upgrading to a 2GB chip. The memory type your laptop will take should still be available from any decent computer service shop.