I upgraded my wife's Dell laptop from XP Home Edition to XP Professional. The computer works except for the web browser. Now, the Google home page won't completely load up. Also, when she tries various websites, she gets the message "Can't Find Application".

Any ideas about what might be going on?

Possibly missing Java?

can you provide us more details on what's going on? What do you mean google doesn't completely load up? What websites do you get the missing application error and what are you trying to do on this website? Like Gerbil said might be as simple as missing java...make sure you have configured your network correctly.

Would upgrading to XP Pro cause us to lose Java and ActiveX? This laptop is wireless with a wireless router. This is new- just installed it last week. Did the new router throw something off?

Bert, does entering this address into your browser get Google to load?: http://64.233.167.147/
"Can't find Application" error? Quite a number run in conjunction with any browser, and one of them could be fouling things up.
If other puters are connecting happily through the router then it is unlikely to be a problem with the router; if the faulty sys is not configured correctly for the router then it is out of my area of knowledge.
Firstly, check in IE [tools, Manage Add-ons] and remove any BHOs you do not recognise or want.
I am sure you have done this.... tried another browser?
Run this tool from Doug Knox - it checks the various IE library files for browsing are correct versions and also registers all dlls involved:
http://windowsxp.mvps.org/IEFIX.htm
Yep, i know it's for IE, but as I said other browsers make use of some of those dlls. You will need the installation cd.
If those things do not work then an infection is a possibility.. some of them interfere with internet access. So...
==Download this file to your desktop: http://download.bleepingcomputer.com/sUBs/ComboFix.exe
- to run it dclick combofix.exe and follow the prompts to start it. When finished, it will produce a log, C:\Combofix.txt - post that log in your next reply.
A word of caution - do not touch your mouse/keyboard until the scan has completed. The scan will temporarily disable your desktop, and if interrupted may leave your desktop disabled. If this occurs reboot to restore the desktop.