HP laptop/XP power supply

I have a 5 year old HP Pavilion laptop running XP (SP2), 2GB RAM.

In the last few weeks the power supply has alternated between AC and battery during use.
this is accompanied by a clicking noise about once/second which coincides with the power icon (plug picture) flicks on and off.

When the laptop is in standby mode it charges the battery (lightning icon on)

It has now got to the stage that I only get 20 mins use before it goes into standby (i.e. it doesn't run on AC power at all).

The battery is as old as the laptop so I can't expect it to last, but it should be able to run on AC power.
The AC adapter is less than a year old and works fine in charging the battery in standby.

I recently switched from Norton (via ISP (BT)) to a free Kaspersky which works really well. Nothing on the Kaspersky site about it affecting laptop power supply.

Starting XP in safe mode with Kaspersky disabled doesn't help.

HP website says reinstall XP (aarghh!).

Anyone got any ideas?