I've posted this before and i will again. There seems to be an issue with avast! Firewall component and uTorrent. Whenever i use both, i get bad mouse lagging and overall OS stuttering after 10 or 15 minutes. But when i remove avast! Firewall (just firewall component) and use Windows Firewall instead, everything is fine even after hours of heavy uTorrent downloading.
And if you're thinking, no i'm not even using P2P Shield...
avast! IS 5.1.864 (File, Web, Network, Behavior, Script, Virtualization)
Windows XP SP3
Acer Aspire One netbook (Atom N270, 1,5GB RAM, GMA950, 500GB 7200RPM HDD)