Antivirus update

AVG Antivirus (the paid-for version, running on Windows XP Pro) started to give me concerns about 12 months ago.  Handling of e-mail in Microsoft Outlook seemed to slow down dramatically, and any sort of activity by the software, such as updates, would make the system grind to a halt.  I wrote to AVG on the subject but the best they could come up with was that the PC seemed to be running a great number of programs, and maybe I could cut it down a bit.
This didn't seem fantastically helpful, so I decided to try a different antivirus and internet security package when my licence renewal came due.  That's what I've done, and the chosen replacement is Kaspersky Internet Security.  Here are my experiences:

Windows XP Pro SP3 (well-configured system, 3 GBytes memory)

Uninstalled AVG and restarted PC, with Internet connection switched off. 
Kaspersky installer tried to check for newer version (even though the package was downloaded two days earlier)
Checking for incompatible software - box was empty - said it was removing incompatible software and then asked me to reboot again. 
Installation continued after reboot and agreeing for a second time to terms and conditions. 
Then I had a Windows error message - Generic Host Process for Win32 Services (?) had fallen over.  The installation then hung on 'deleting backup files' - cancelling the installation seemed to do no good, and in the end I did a hard switch-off, holding down the laptop's power-on/off button.
After restarting, lo and behold, Kaspersky antivirus was installed. Reconnected to Internet.  I entered the activation code, and then set off the update process.  This took ages - something like 150 MBytes to download.  More annoyingly, because this wasn't part of the original install package, I have to do this again for each of the other two PCs included in my licence.
A few add-ins (URL advisor was one of them) needed to be checked when loading Mozilla Firefox.
Still getting used to this on Outlook.  The set-up for spam detection is different, and I'm not sure it's even as 'quick' as AVG when clicking Next Item.  My first feeling was that incoming mail scans seemed to be slower, and I also felt that the PC was less responsive, hanging for a second or three when switching from program to program or window to window using ALT+TAB. Now after a few hours' use, I'm feeling fairly comfortable with it. Maybe the problem was the first run syndrome - sometimes settings need to be made on the fly, and this can slow everything down.

Update 20/04: Had to reboot again following installation of Kaspersky updates.  Seemed slow again when starting Outlook.

Update 26/04:  Multiple occurrences of system becoming unresponsive for minutes on end.  Outlook 2003's CPU usage doesn't just spike, it goes to 100% and stays there.   This seems to be triggered by double clicking an e-mail in the Unread Messages list - a little fundamental, and can be overcome by switching to another (non-Outlook) window.  I've been searching the support forums for a solution, but found nothing so far.  This is causing me to look urgently for some other anitvirus or Internet security package. 

Windows Vista SP1

Uninstalled AVG and restarted PC
Kaspersky installer installed without glitches or need to reboot again.
I entered the activation key and then was able to kick off the update process.  As mentioned above, 150 MBytes.  Then it seemed to sit on 'updating databases and the application (88%)...' for ages, but finally got there. 
This is a Windows Vista system, so I don't use it for serious work, but as an occasional standby and browser host.  I don't run any e-mail software on it, so I can't compare with the XP Pro experience.  Experience so far is that IE9 is fine, but Mozilla Firefox crashed repeatedly when trying to access the BBC News site or iGoogle.  Not too clever, a reboot seems to be the next thing to try.
After a reboot, Firefox seems to be working fine.  And there's a Kaspersky gadget in the sidebar, slightly bulky but useful nonetheless.

Windows XP Pro SP3 (small memory system)


Installation steps were simple, as for Vista - it didn't ask for additional reboots.  Again there was a large download to update after installation.  I'll be waiting for feedback from the user of this system, with fingers crossed.

Summary

Installation was fairly painless apart from the first install which was done without Internet connection.  I don't like the fact that I had to download the updates three times.  That's really wasteful.  The glitches with Firefox seem to have been resolved by a reboot.  And I've also installed the Kaspersky virus protection product on my MacBook Pro, to see how it goes and if anything is detected. Overall I was starting to feel good about the decision until problems with Outlook came to light.  Now trying to find out why, and what I can do about them.