|Installing XP after Win7 is a bit tricky, plus if you have a SATA HDD, XP will NOT recognize it unless you take one of 3 steps:|
1. install the SATA drivers at the start for the XP installation by pressing F6, then loading the drivers from a floppy disk
2. change the SATA setting in the BIOS to IDE mode
3. create an all new XP installation CD with the SATA drivers slipstreamed into it.
But as guapo pointed out, before even attempting to install XP, you need to create a partition for XP to install to. You can't do it from the XP disc, it has to be done in advance using a 3rd party program such as Easeus Partition Master. Get the free version, it works great.
It would help to know your system specs, I'm guessing they're not "XP/2600-1024/133? You mentioned a it's new HP computer? If it is, it probably already has the max number of primary partitions (4), so to be able to do anything, one of those partitions has to go or has to be converted to logical. Here's some tips:
BTW, have you tried running your programs in XP compatibility mode?