Did you remove the entry for your previous card from Add/Remove Programs?
WinXP also has the "Safely Remove Hardware" utility
(there's usually an icon in the systray by default)
This would be the equivalent of Removing the entry from Device Manager.
Most video card manufacturers recommend removing all traces of previous cards before installing new ones.
As for the conflicts, please could you post a full IRQ list here?
You can copy+paste this from the System Information Tool.
The IRQ's can be changed in the BIOS usually, depending on your mobo and BIOS.
Press DEL when your system boots up, before the Windows Welcome Screen appears. Depending on your PC manufacturer/model, Setup might be entered by pressing a different key, eg. one of the F keys. Consult your mobo manual for full instructions.
eg. IRQ's for PCI Slots/cards can be changed via:
PnP/PCI Configuration
If you don't see this option, then simply moving a relevant pci card (modem, nic, soundcard, etc) to a different slot will change the IRQ.
Any unused onboard/integrated devices can be disabled via:
Integrated Peripherals
You can't change IRQ's in ACPI mode from within Windows, and to be honest, standard computer setting isn't recommended. In standard mode, if Windows will let you (it won't allow it for some devices), you can change IRQ for a device via:
Device Manager -> Device -> Properties -> Resource tab
Hi-lite IRQ
Uncheck "use automatic settings"
Click "Change Setting"
However, I still recommend you use the BIOS method instead,
and all relevant system specs & that IRQ list will definitely help.
[edit]
Unless the graphics card manual specifically states that you should change any BIOS settings (Advanced Chipset Features -> AGP Timing Control), then usually the default AUTO settings
should suffice.
Useful Articles:
http://support.microsoft.com/default.aspx?scid=kb;en-us;314068
http://support.microsoft.com/default.aspx?scid=kb;en-us;315278