How to clear the windows security log?

Note: You need to be an administrator user to perform the following operation.

Control Panel > Administrative Tools > Computer Management

Now select 'Events' on the left hand side. Right hand side you should see 'Security', right click on it and choose 'Clear all evnets'.

How to remove the Picasa synced photos from Android?

Force stop (Kill) the Gallery App and disable the data transfer.

Go to Settings >  Accounts & Sync > your Google account and uncheck the Sync Picasa Web Album option.

Now go to Settings > Applications > Manage Applications > Gallery and clear the cache & data.

Enable the data transfer, open the Gallery again, hopefully you should be good.