I have 1Gig of memory in my machine & WinXP runs like a dream unless I do something stupid, I haven't had it crash for more than a year now since I stopped doing stupid things to it. It does sound like your harddrive needs defragging as both Vue & poser get a little tetchy when the HD gets frayed round the edges. If it's Poser4/PP you're using they both have problems with large amounts of memory but there's a fix for both versions on the CL website, Vue4 is known to be a little on the buggy side too.

Windows 7 64Bit
Poser Pro 2010 SR1