I bought a used CPU with a Celeron 1.0 and XP Pro. The seller put a 4g hd in place of the orig. I have a 13g hd and I want to use the 13g as the master and use the 4g as a slave with the OS. The 13g is clean with a new format. Is this possible? I have ran two hd's in the past, but the master has always had the OS. When I have tried this one that way (4g as master) it gets full of stuff from temps, antivirus, and firewall programs. This really seems to slow it down. Any help?

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The easiest way would be to let that 4GB remain as Master. Install the 13GB as slave.
Then uninstall some bulky programs (AV, Firewall, Office etc.) from the 4GB and install them on the Slave instead. Change the location of "My Documents" from c:\documents & settings\[userid]\ onto somewhere on the 13GB drive (you can use M$ TweakUI for XP for this). That should give you enough breathing space.

Real.. is right (usually is) you can move the Tempory internet page file and OutlookEx data file to the larger drive also.They build up with a lot of Data.
Also ,my memory escapes me ,but just in case ,check the ATA speed for those two drives.If the 13 Gig is 66 and the 4Gig is 33, the 4G as master will force your machine to default to 33.