Browser will not load certain pages ...

All,
Got a maddening problem, and looking for suggestions.

My daughter has an XP machine, and until recently, she routinely
visited www.picnik.com with no problems. She recently sought help
from our household IT professional (me) because "it was not working".
Here's what I know:

1) Not browser related. Have installed/uninstalled one-by-one Safari,
Chrome, and Firefox. All load the same common websites (Google,
Facebook, etc). All resolve Picnik URL, and appear to start loading,
but never finish.
2) Is machine related. Can load Picnik easily on Win 7/Chrome and
Ubuntu/Firefox machines. All machines connect wirelessly through same
router.
3) Not network related. Thought it could be the WLAN NIC and
bandwidth, but disabled WLAN and wired directly to router. Same
behavior. Good bandwith (about 1.3 Mb/sec) confirmed by downloading
large files.
4) Not Flash (Picnik is Flash). Other Flash sites working fine.
Reinstalled Flash to no avail.
5) Cleared cache, history, temp files, etc. About 20 GB available on
80 GB disk.
6) Seems to be high, rhythmic disk activity. HDD light on for one
second, then off for one second, over and over. Maybe a big swap
file?

Looking for ideas,
Chris