Weird BSOD

I have a strange problem with my XP installation. Once a day fairly soon after starting up in the morning (but not at a constant time after), my machine crashes with a BSOD. The message is mostly ntfs.sys address B9E35A54 but I have had other messages about attempting to execute a non executable part of memory (something to that effect). The comp is happy for the rest of the day after that. If I happen to need to switch off at some point in the day, on switching on again the same thing will happen just once.