The home version is more than secure enough for you. The added benefits of pro are for a networked enviornment.
You real problem will be that your PC will not run well with XP - 433mhz and 256mb RAM may well exceed minimum requirements but they will not make for a satisfactory computing experience.
Do you have a problem with WinMe? A reinstall of WinME would help a lot.
Even a "downgrade" to Win98SE would be a positive step.