There are a few things that may cause this.
1) 99% of the time, this is caused by the Windows XP built in firewall being enabled. The CMS needs to be able to access the client machine. You can disable the Windows XP firewall by going to the control panel on the client machine and selecting Windows firewall. Make sure it is set to 'Off'. Or, alternatively, you can allow the file and printer sharing in the 'Exceptions' of the firewall settings.
2) The machine may just have been shut down, or is rebooting, and CMS hasn't realized it's gone offline yet. It will usually stop trying to install shortly.
3) If the machine is in hibernate mode, it will show up in the network browse list, so CMS thinks it's online. However, when it tries to connect to it, the machine won't respond.
4) Verify that the client can ping the CMS and see the CMS's VB_config and VB_install shares. Also, check that you can browse to the C$ share of the client from the CMS machine.