My laptop with Win XP has been given IRQL error with following STOP message:
"0x0000000A (0x400005E1, 0x00000002, 0x00000000, 0x804D9B64)"

I have not installed any new hardware or software recently.

Any help will be greatly apprecaited.

This may indicate a failing hard drive...

I have a failing HDD in my second PC that gives the same blue screen error (IRQL_NOT_LESS_OR_EQUAL) randomly. It takes ages to get started up. I'm going to have to replace it, which is no big deal as it's pretty old anyway, and doesn't start up or starts up after a long delay at boot sometimes. Yesterday I had a failure of lsass.exe, at boot, and the system shut down. Usually this type of shut down is initiated by "NT_AUTHORITY\SYSTEM" on this occasion it was just "\".

I had that too. In my case it was caused by a faulty Ram chip. And the faulty Ram chip was caused by an overvoltage in its slot on the motherboard.

Try to use memtest86+ to see if an error pops up. Do at least 7 or 8 passes. (shouls take about 45 minutes)