To install the HP DesignJet 750C XP 64 printer drivers from a XP 64 Installation DVD using the Add a Printer dialogue:
1. make the appropriate local/network printer selection,
2. make the appropriate printer port selection,
3. Use the Have Disk Button
4. Browse to the DVD drive/AMD64 folder and select the DRVINDEX.INF file.
All the XP 64 printers, including HP printers, should be accessible. The files for my HP 750C+, besides the DRVINDEX.INF file according to the Vista Windows Printer Test Page are: PLOTTER.DLL HP75036.PCD PLOTUI.DLL PLOTUI.HLP. All of these are located in the AMD64/DRIVER.CAB file.
I haven't tried to install or use the HP DesignJet 750C in a XP 64 installation, however I have used this procedure to install the HP DesignJet 750C drivers in a Vista 64 installation and in a Win7 64 Beta installation. I regularly use the HP DesignJet 750C for monochrome plotting from vanilla AutoCad in Vista 64. I didn't test the Win7 64 installation except to monochrome print the test page which worked OK. Also, the HP DesignJet 750C drivers were updated in XP 64 SP1.