1) Your hardware is too old for WindowsXP. The Pentium II came out before Windows98... about 4 years
before XP. While Windows98 is very out-dated and such - it was something the PII can handle.
2) 128mb is painful. Memory for a P2 should be something like PC133. Go to criagslist.com and you maybe able to get that for free. I think I have some in a drawer somewhere and I'm tossing stuff like that into a recycle box for an electronics recycle center. Getting up to 256 or 512mb will help. But the CPU, motherboard, horribly slow memory will not make much of a dent.
3) I'm not rich either. I get parts when I can. But don't spend a dime on what you have. I've given away 700~1000mhz P3 systems because they have so little value. If you look, you may find at places like Microcenter that older P4 Dell & Lenovo(IBM) refurbished computers for about $180~250 with WindowsXP installed.
4) Don't worry about Vista. If your current PCs are running fine with XP, keep them there. Vista requires much more memory and CPU power to be usable.
5) Dell isn't the best choice for a low cost PC. If you buy a NEW desktop - it will most likely come with Vista. If so, make sure it has at least 2GB of RAM. Here are some for about $400.
http://www.microcenter.com/single_product_results.phtml?product_id=0291231 Getting a low-end Intel CPU with 1GB with vista is a horrible experince.
6) If you get something on the weekend, you can usually get it with an LCD monitor and printer for about $500~550 (again, 2GB - AMD x2 CPU).