Sudden reboots sometimes happen when you have a bad, old or over-burdened power supply .
This is the only reson I can think of causing random rebooting .
If this is the case then it's not Windows XP it's fault .

I have heard about Windows XP deleting your "My Documents" folder if you have multiple profiles though .
And I mean there really gone !
Booted up from Win98 DOS disk and couldn't find them on the HD's anymore !