PHP Fatal error: Allowed memory size

[Aeneas]

"PHP Fatal error: Allowed memory size of 262144 bytes exhausted (tried to allocate 334676 bytes) in Unknown on line 0"
What is causing this failure of Apache after 2 seconds ?
Apache was launching fine on the previous boot, and nothing much has changed.
Q6600/6 GB Sdram/Windows Vista 64 Ultimate