Raid rebuilding after downloads

[NOD]

Problem is that after a download/update of any software , the next time it is fired up, it will rebuild channel one of the set.
usually this will take, seems like 10-15 minutes, until the computer is usable again.
Any help please.

System
Gigabyte GA-7NNXP motherboard
BIOS F16
AMD Athlon XP 3200+ @ 2.19Ghz
1.0 Gb Ram
2 x Western Digital 36Gb Raptor Serial ATA HD
Windows XP Home Edition SP2
Settings
Silicon Image SiI3112 SATARAID Controller
- look ahead enabled
- write cache enabled
- transfer mode Ultra DMA Mode 6
windows settings
Virtual memory set to 1534Mb, on "D", No paging file on "C"
HD = C, D, E. 18002Mb/10001Mb/7287Mb
SCSI Properties
- disable tagged queuing
- disable synchronous transfers
write caching set to "optimize for performance

Thanks Nod