No, even 256 Mb is not enough to run Windows XP. Also, it depends which memory do you use. Is it still SDR or DDR, slower or faster ? 512 Mb is recommended by many for smooth running of Windows XP, and believe me even that sometimes isn't enough. For some graphical demanding operations, especially if you need to have more than 5 huge programs open in the same time (CorelDraw, Photoshop, Fireworks, Dreamweaver, Flash etc.), going below 1 Mb of RAM is waste of your as well as porcessor's time.
256 for sure is not "ideal", it's not even "optimal". "Optimal" could be 512, but as we all know... never enough RAM.
It's well known that Windows XP takes forever to load with 128 MB of RAM only. As soon as you upgrade your computer with more RAM, you'll see noticeable increase in speed... not just loading of Windows, but also, processing some tasks, and many other operations.
Cheers !