XP getting restarted after login screen appears

i think it is time to start backing up your data..
have you checked if it is hardware issue?
did you installed any new hardwares recently? or even updates for system or drivers?

i had done shutting down the computer normally but still the same problem exist.

Gotcha found in Event viewer details follow

The RAM Driver service failed to start due to the following error:
The service cannot be started, either because it is disabled or because it has no enabled devices associated with it.

Source : Service control manager

Event ID: 7000

I think it is embarrassing problem for few users just copy this in google you will find people are still helpless to resolve this issue, I still doubt it is some trojan or virus got hidden into system.

Any suggestions will be appreciable

here is microsoft's solution for event id 7000
support.microsoft.com/kb/314357

i had worked out on the the following step one but no use, i tried to attempt second step (method 2) and i'm unable to understand the sentences saying Double click the services, In the right pane, right-click the appropriate service, and then click Properties.

On which appropriate service microsoft wants us to click, what is that service name ?

More suggestions will be highly appreciable.

As per the microsoft :

On a Local Computer
If the user is on a stand-alone Windows XP-based computer, follow these steps:
Log on to the computer as administrator or as a member of the Administrators group.
Start the Local Computer Policy snap-in.
Expand Local Computer Policy, expand Computer Configuration, expand Windows Settings, and then expand Security Settings.
Expand Local Policies, and then click User Rights Assignment.
In the right pane, right-click Log on as a service, and then click Properties.
Click Add User or Group, add the user that you want to the policy, and then click OK twice.
Close the Local Computer Policy snap-in.


Method 2: Configure Logon Information for the Service
If the password is changed on the user account that the service uses to log on, configure the password to match the current password for that user. To do this, follow these steps:
Click Start, click Control Panel, click Performance and Maintenance, and then click Administrative Tools.
Double-click Services.
In the right pane, right-click the appropriate service, and then click Properties.
Click the Log On tab, type the correct password in the Password and Confirm Password boxes, and then click OK.
Right-click the service, and then click Restart.
Quit the Services tool.


Method 3: Configure the Service to Use the Built-in System Account
If the service still does not work with the specified user account, configure the service to log on to the built-in system account. If also the Upload Manager service does not work after you uninstall Service Pack 3 for Windows XP, configure the service. To do this, follow these steps:
Click Start, click Control Panel, click Performance and Maintenance, and then click Administrative Tools.
Double-click Services.
In the right pane, right-click the appropriate service, and then click Properties.
Click the Log On tab, click Local System account, and then click OK.

NOTE: In most situations, it is not necessary to configure a service to interact with the desktop. You do not need to select the Allow service to interact with desktop check box.
Right-click the service, and then click Restart.
Quit the Services tool.