I normally leave ZD2 minimized and check my e-mails whenever the yellow envelope pops up in notification area of the taskbar of Win Vista.
I rarely shutdown Win, but rather hibernate.
The following happens quite often:
1. In any situation:
a. A quite huge chunk of the the system memory is taken by the zdclient.exe process (often about 1GB!!!!!!!!!).
b. From times to times, ZD just stops working and an error message informs that prism stopped working and I have close and to restart Zimbra.

2. Upon restoring the system from hibernation, the computer is often unbearably slow and only when I kill the zdclient.exe and zdesktop.exe processes it comes back to life with decent performance!

System:
Windows Vista Business SP2 32 bit
Intel Core2 CPU T7200 2x2.00 GHz
RAM: 2GB
Sony Vaio VGN-SZ5XN

Thanks!