Yes, you normally can if you have more than one SATA controller on your board and you have a spare IDE drive
1) You keep your HD plugged into one of the non-RAID SATA ports.
2) Enable the RAID on the other ports in the BIOS.
3) Boot into you OS and install the driver for the RAID controller.
4) Reboot. Run a chkdsk, reboot, degrag, shut down.
5) Ghost drive from orginal SATA to spare drive. (My have to use -noide switch on GHOST
6) Connect both SATA drives to SATA RAID ports.
7) Boot into RAID BIOS on the controller, create your RAID 0 Stripe. (You may have to initialize it after, depending on controller) Reboot
NOTE: Step 7 WILL ERASE ALL DATA on your original drive! Make sure your spare is a good drive
8) GHOST from spare to your new stripe (again, may have to use -noide switch)
9) Shut down, disconnect spare, boot up
10) Reboot one more time, after Windows has installed the new drives in Device manager. DONE
Now, if you only have one controller on your board for SATA OR RAID, you will have different steps.
1) GHOST to spare IDE drive with -noide switch (chkdsk and defrag first).
2) Disconnect SATA drive.
3) Change SATA to RAID in BIOS.
4) Boot from IDE spare and install SATA RAID drivers.
5) Boot up and shut down to make sure RAID drive is installed.
6) Goto to previous instructions, step 6
EDIT: Hard drives are cheap and RAID 0 will give a huge boost when booting, transferring large/lots of files, recording video and most importantly - loading games