I have an 'antique', a Compaq Armada 7400 laptop, with a PII-300, 256MB memory and a 6GB harddisk. I installed XP-Pro in the 'corporate' version that Halo71 mentions, and it runs like a charm. Can't say it is slow, because there is nothing else on it, other than Word 2000, Excel 2000 and EditPadPro for making HTML-changes. No Antivirus and no Firewall. I have a PCMCIA-NIC for it, but I'm not connected/-ing to the Internet.
I only use this laptop to show website-designs to potential new customers and it works just fine. I still have over 4GB free on the HD.
My daughter used this laptop as her main computer until recently, running W2000 and Office 2000 and it had only 128MB. That laptop was on 24/7 for nearly 4 years! Her fiancee bought her a 'proper' PC so she gave this laptop to me. I just bought some more memory and I'm pleased as punch with it.
This should answer Halo71's question.