As I was advised by the UBUNTU itself to update the kernel, I did so.

But after updates, I can see the following problems:

1. It's taking more than 10 minutes to get into the desktop after typing username & password.
2. System has gone very very sluggish.
3. It's taking 5 to 7 minutes to open any application (be it Terminal or Firefox or RhythmBox or whatever...)

My system specs are:
-------------------------
ASUS P5GC-MX Motherboard.
1 GB DDR2 RAM
NVIDIA Video Card of 256 MB Onboard memory
PCI Sound Card of Frontech

Please somebody explain... and advice me what I can do...??? :0

Everything was fine before the update!!!

Is it virus? (I heard that LINUX has no Virus; basically this is one reason I have started using UBUNTU Fiesty Fawn kernel ver. '2.6.20-15-generic'

Maybe some conflict with a driver? If you installed one or more drivers manually, they need to be reinstalled after each kernel update. If you have GRUB installed, you can always boot using the older kernel (Ubuntu always keep an older kernel around to protect against issues like the one you're facing now). If you want to use the newer kernel, you'll have to find out what is going wrong. Do you get any error messages? You can use the command dmesg, which will more than likely show where the problem lies. If you can't make any sense of it, you can post the output (all of it or just the error messages) here and I'm sure that someone will have a look at it.