I was curious myself if Windows has an "in-house" way of doing this. Apparently, a part of SCCM, called Data Protection Manager partly offers what you are looking for, if I understand it correctly. That requires you have much more enterprise infrastructure than just a Windows XP box, so I cannot be certain that is even an option.
More in your ballpark, but still requires some infrastructure, is modifying a Group Policy, if you have an Active Directory Domain, is modifying Group Policy settings under Computer Configuration -> Administrative Templates -> Network -> QoS Packet Scheduler -> Limit reservable bandwidth. As the policy setting description states:
At least Windows XP Professional or
Windows Server 2003 family
Determines the percentage of
connection bandwidth that the system
can reserve. This value limits the
combined bandwidth reservations of all
programs running on the system.
By default, the Packet Scheduler
limits the system to 20 percent of the
bandwidth of a connection, but you can
use this setting to override the
If you enable this setting, you can
use the "Bandwidth limit" box to
adjust the amount of bandwidth the
system can reserve.
If you disable this setting or do not
configure it, the system uses the
default value of 20 percent of the
Important: If a bandwidth limit is set
for a particular network adapter in
the registry, this setting is ignored
when configuring that network adapter.
This also requires you get intimately acquanited with WMI filters in said GPO's for your time requirement, but I am not sure they are that reliable (in terms of on the dime processing at a given time, since refresh rates, IMHO, are quite variable in larger, poorly-maintained domains common to big companies like I deal with).
That being said, these are bizarre solutions. I modded up the NetLimiter answer, but is anyone doing anything like this in a big enterprise environment? Would love to hear stories if it works well.