Problem with booting in Windows XP

My computer has Windows XP and WIndows 98 installed under dual boot configuration.Windows XP is installed on the active partition and WIndows 98 on the second partition.Recently I increased the capacity of RAM from 64MB SDRAM to 128MB SDRAM. There weren’t any problems,but lately by PC keeps on rebooting and ‘am unable to get in to WinXP. But i am able to log into Win98.
What could be the problem?