Xp blue screen error

i have already posted this problem about this..but here are more details..i have multiple os(win98,win xp)…my win98 seems to work perfectly…my xp always shows some sort of a blue screen error which says this..

“a problem has been detected and windows has been shut down to prevent damage
pfn_list_corrupt”
technical info:
***stop:0x0000004e(0x000000002,0x0000b644,0x0001feef,oxoooooooo1)

..i recently installed a new 512mb ddram..i had a 128mb sdram…my comp is equipped with intel p4,40gb hdd..please suggest the necessary steps tht should be taken