Duron 700@700 with 20 GB 7200 rpm hard drive and 128 MB RAM (unless you're going Windows 2000 - then I'd consider 256). 32X CD-ROM, Lucent Winmodem. 17" Samsung monitor. SB Live! Value. If she doesn't play games then an ATI Xpert 2000 Pro. If she does then a Geforce 2 MX or even just a Voodoo 3. I'd get a cheap Linky or D-Link NIC (US$15) even if she doesn't play on using it, just in case.
That very cheap system is better than 99% of the computers in my workplace, all of which run Office just fine. Good browsing I think needs a Celly 400+ though.
By the way, unless you get the parts wholesale, it might be cheaper to get the local store to build it for her, and you get the extra warranty to boot.