I have a Dell Opti Plex GX1 desktop,and attempted to reinstall the Windows XP Home addition from my orginal CD,as the setup reached the section where it loaded the XP setup from CD,I hit enter and a screen came up  that no hard disk were installed on my comp,and to make sure that any hard disks drivers were powered on and properly connected and disk related hardware configuration is correct.Also said this may involve running a manufactures-supplied diagnostic or setup program and setup couldn't continue.I'm very dumb when it comes to computers.Please tell me what I will have to do in order to possibly get my computer back up and running.A GX1 - ALLRIGHT!   Please post the service tag so I can tell which model. It is on the side of the case or in the BIOS. Press F2 when you see the Dell logo and see what hard drives are LISTED as well as the service tag. After you see this, press ESCAPE to exit the setup. When the machine starts again, see on screen under the Dell logo for the BIOS revision.

I'll wait for the info.  Glad to see somebody will help.The service tag # of computer is 07YF5,and bios revision is A04.The bios reads as follows;
Diskette Drive A-3.5 inch,1.44MB
Diskette Drive b-Not installed
Drivers:
Primary Type CYLS  HDS  PRE    LZ    SEC  SIZE
Drive 0:   1      306    4     128   305   17     10
Drive 1: None
Secondary
Drive 0 : Auto                CD - Rom Device
Drive  1: none
Reserve Memory         None
CPU Speed                 400 MHZ
NUM Lock :                  on
Chassis intrusion          disabled
Dac  Snoop :               Off
ACPI :                        Off
That machine was manufactured 4/9/99 and came with a PII-400, 6.4 gig hard drive and 64 meg RAM. Hopefully some of that has been upgraded. How much RAM currently is there? How large is the current hard drive? Did you just install it?

The thing that jumps out at me is that hard drive needs to be set to auto rather than specific heads and cylinders. When you do that it should tell you the size of the drive as well.

BIOS A04 is also rather dated and you may need to update that to access the full size of a newer drive. The newest BIOS is A10 and can be found here.

http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R31962&SystemID=PLX_PNT_PII_GX1&os=WW1&osl=en&deviceid=162&devlib=0&typecnt=1&vercnt=6&formatcnt=2&libid=1&fileid=34958

You just download the file and then double click on it with a blank floppy in the drive and after it is created then restart the machine wih the updated BIOS floppy in there (it will walk you through it). It takes about 30 seconds. Do not power off the machine during this. Other than that it is very safe. When the machine is restarted is will show BIOS revision A10.

So do that and let me know the size of the drive after you do all of this.  

Thanks for all your expert advise,but I finally gave up and sent this comp for repair.The repairman said my switch was missing on my hard drive,guess last tech stole it.I replaced it and can access with fast access internet,but can't access with DIAL up service.Repairman said I had a bad modem.It this a costly repair?Any suggestions on how to repair modem other than replace?Thanks for all your help,as this is a great site with techs that are very good and have nerves of steel.Thanks for any suggestions.A replacement modem can be had for $10-15 new and replacement involve removing one screw, removing the old and putting the new one in, replacing the screw, starting the machine and installing the drivers from the CD thant comes with a new modem. Total time less than 10 minutes.  

http://www.newegg.com/Product/Product.asp?Item=N82E16825104001

as an example.Thanks for your help,but unsure which modem I will require for the GX1,opti plex.Can you please tell me.
Thanks,Any dialup modem will work. A real harware controlled ISA modem would probably be fastest, but the cheaper WinModems will work satisfactorily wilth that processor, like the one I linked to. Check the details for the indiviual modem that it will work with a PII-400. THe requirements will be in the detailed specs in the listing or on the side of the box if you go to a store. A GX1...Excellent !