Look, all you guys sound like you never seen a computer with less than 4 GB RAM :P
Actually, I'm sure that with as much as 1 GB RAM and a SSE-enabled processor (something like single core old) you already can use SAI.
In any case, when I wanted to spring some life in a very, very old computer to turn it into a relatively OK media machine, I used Linux. The thing had a 400 MHz processor, that's right, 0.4 GHz, 128MB RAM and a TNT nVidia card, and it could stream web media. It booted in like half a minute, but that's not too bad for those specs.
I'm not saying it's wonderful to use an old computer, I'm just saying that if you put Windows 7 (or even some heavy linux like Ubuntu), it will be absolutely unbearable. And the machine, as old as it is, can do much better.
For example, there's a tiny Linux distribution called SliTaz.
http://www.slitaz.org/en/If you install it into a USB drive (can be a thumb drive, it's VERY small), you can boot from USB, use it (you will be amazed at how fast it is), and before turning it off you can choose save changes (or start anew next time). The system itself runs in even something absurd like 64MB RAM (Actually I've tried that in a virtual machine, and it's true).
This way, you can run Gimp with pressure sensitivity (requires installing the drivers, I believe they have Wacom somewhere in the name so searching is easy).
SAI works in Linux, if you install Wine (a Windows compatibility layer that allows you to run Windows apps in Linux). Actually, it runs almost flawlessly, the only thing not working is pressure sensitivity. Photoshop CS5 barely works in this mode, but IIRC it's useable to the extent of applying filters and such. Paint.NET might work just fine if you install Mono, which is an open-source implementation of .NET.
In other words, you can choose to ditch your broken laptop, but there's a big chance that it's still perfectly usable for some tasks. And in a moment of despair, Linux will always be there to give you a hand :P
Also, I have absolutely no idea if you can do what I'm going to suggest, but there's an obviously unsupported and unlikely legal way to boot Windows from a CD, this might, MIGHT allow you to install Wacom drivers and use SAI instead of getting used to Gimp (not sure if you've used it before). Look for Hiren's boot CD, download it and burn it. It has modified versions of Windows XP and 7 that boot to RAM.
Another thing you can do is snatch a hard drive from eBay/Amazon for less than $50 bucks.