Buddy, I still remember what Ca Bhupati told during our passing out ceremony! The next best thing you can invest is on a laptop...
Since your use is limited to tally, excel and word any good branded laptop with the following configuration will help you
1. Processor - Core 2 Duo (i3/i5 is not necessary as tally/ office will work superbly with core2Duo)
2. 2GB Ram - minimum (if you are taking 32 bit OS - no need to go for 4GB as 32 Bit uses only 3GB max - check before u buy).
3. Harddisk - 250GB plus - higher the better since you may have to store a lot of data.
4. 14inch plus screen - since tally and excel requires a larger screen - personally i am using a 13inch lap but sometimes feel like having a 15inch - esp while using excel.
5. webcam/mike/other accessories are part of any laptop deals now a days - they will give it free or lap will be having inbuilt cam and mike. try to get a bluetooth mouse - very handy and will free ur valuable USB port. Am using a logitech one - not sure whether available in India.
6. lenovo - I dont recommend. Sony - very costly for the product u r getting. Toshiba, Dell, Acer - good, price wise as well as product wise. My wife using Acer for the last 3 years - no problem.
7. Since the new office 2010/ 2007 works better in Vista/ 7 machines, I recommend windows 7! Else XP will do the trick for you - tally/ office will work in that.
8. Good antivirus - always buy antivirus software - do not use pirated one..
9. A good external hard disk - Westerndigital or toshiba or seagate or hitachi - 500GB plus. This is a must and make sure to back up ur important files often. If your data is gone or laptop stolen... this will help u.
10. Unless u r not going for gaming and all... no need for dedicated ram. Bcoz, that will increase ur lap cost by 100$!
11. It is recommended not to use ur official/ office lap for personal, gaming and downloading purpose.
Thats all... cheers...