I use Paint Shop Pro X2. I have no problems other than with Alpha channels now and then. Even the .dds Photoshop plugin works with it.

Windows XP SP3
Intel Dual Core CPU: E5200 @ 2.5GHz
4.00GB DDR3 Ram
ASUS P5G41T-M LX
PCIE x16 GeForce GTS 450 1Gb
SB Audigy 4
Spyware Doctor with AntiVirus