Something I have noticed

Win Xp 32bit using the same library as on my win7 64bit
After starting, calibre was using 135734 K full screen
Minimise and calibre was using 4638 K
Maximise and calibre was using 10876 K

On win7 64 bit
minimising and maximising there is no change
calibre is now using 231036 K

It may be something on Win7 64bit as I use O&O clever cache 64Bit
Which is for controlling memory
It says that it has recoverd 112MB but the program is now using 394MB
When it is started it is using around 22MB but this keeps growing and growing.
O&O clever cache has not been running on win7 whilst I was getting readings
But was running on Win XP so it is not O&O clever cache
Both machines run BT Netprotect Plus by McAfee does not look like it is the AV

Win7 64bit
I have just switched to a small library of approx 500 books calibre has now gone up to 266376 K