Is it safe to keep system horizontal ?

HDD, yes.
DVD writer, check the manual if vertical position is supported.

Actually when I keep my sys in normal position it suddenly restarts in between , while in horizontal position it doesn't ( or rarely ). My DVD writer is SONY DVD RW DRU-710A. Also my SMPS fan makes weird sound when I start my comp ( like a helicopter ) wat cud b the issue ???

is it coz of SMPS ????

If the SMPS fan is making a weird noise then the problem is obviously in the SMPS.

Change your SMPS to something better.........Try to get a branded one n atleast 450 to 500 W

wat r u saying man ??? I think Cooler Master 380W for 1500 Rs will be Enuf for me. My sys is ::
AMD AThlon 1700
Asus Mobo
2 HDDs
1 DVD writer
1 soundcard
2 LAN cards

Cooler Master 380W is more than enough for your config.

what abt 400 W VIP or Powersafe. Cost is 1500 Rs. Cooler master is xpensive 380W 2100Rs.

my friend has 500Mhz PIII its a HP PC
(dont know wht does HP mean hewletter Packward or Horizontal PC)
he keeps his PC horizontal also his CD drive is fixed in horizontal Position
it 6yrs old PC

There is absolutely no problem in keeping the system horizontal, provided you have the space.
The DVD drive you have can hold the CD while you are opening or closing the tray.
Regarding the SMPS, if u can, then go ahead and open up ur SMPS and clean the dust accumulated in the fan and over the board.
There are some tutorials for oiling the noisy fans.
You can find it here

thanks for the reply junta but I wanna know abt the SMPS price and quality.Is 400 W VIP or Powersafe is gud Enuff ??? Cost 1500 Rs