I'm lazing at this and I'm offering other Experts to jump in an provide a nifty inexpensive solution.
Current network system...
4 XP Home Workstations
1 WIFI SRX router with 4 - 10/100 ports. 3 available
None of the systems are sharing files and folders. However, there is a need to backup. Most efficient and cost effective comment(s) are being sought. Keep name brand in mind, avoid "dirt cheap", and must have product support with good warranty.
Solution MUST NOT INCLUDE server.