I have modified following files in Magento
app/code/core/Mage/CatalogSearch/Block/Advanced/Form.php
app/code/core/Mage/CatalogSearch/Model/Advanced.php
Please suggest me how to overwrite the core files of magento. I have already created a module named 'Custom' for another purpose. So to overwrite the catalogsearch folder first i copied the Form.php file into my own module’s(custom) Block folder and created same structure as magento does. I have renamed the class name and it extended Mage_CatalogSearch_Block_Advanced_Form. I also added the following code in config.xml file inside my module/etc folder.
<global>
<blocks>
<custom>
<class>Company_Custom_Block</class>
</custom>
<advanced>
<rewrite>
<form>Company_Custom_Block_Advanced_Form</form>
</rewrite>
</advanced>
</blocks>
</global>
But it didn't work for me. Please suggest me the needful.Thanks!