The software that comes with USB KVM switches usually is for loading switch-controlling software on xp systems, not to make the USB switch functional.
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My biggest complaint with USB KVM switches is the redetect delay when you switch between systems. You may have to wait 5 sec before you have full keyboard/mouse control after switching.
The positive side of using a USB switch is that you can boot windows without having the USB KVM switch pointing to the win box and later get full mouse/keyboard control. If you’re using a PS2 mouse/keyboard combo and windows doesn’t detect them during the boot (i.e., the KVM switch is pointing to a different system), it is likely that they will not work after the boot is complete.
No experience with a DVI KVM switch, so can’t help you there, but I wouldn’t expect any problems.
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Haven’t found a USB KVM switch that I can strongly recommend, probably because I prefer the PS2 versions. You usually get a little more for your $$$ with a PS2 switch.
Currently, my personal favorite PS2 switch is the IOGEAR GCS614A for 4 systems (I own 2 of these), which includes attached PS2 cables and stereo speaker connections. US$69.99 @ newegg:
http://www.newegg.com/Product/Produc...82E16817107419
For a more conventional setup, I use the PS2 Linksys ProConnect series switches, which come without cables. You can daisy chain these switches together and form a switch array. LinkSys ProConnect 4 system SVIEW04 (I own 2 of these) US$101.99 @ newegg, but currently has a US$50.00 rebate (US$51.99 after rebate):
http://www.newegg.com/Product/Produc...82E16817107748 Warning: Good cables can cost as much as the switch.